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NAME
       perlretut - Perl regular expressions tutorial

DESCRIPTION
       This page provides a basic tutorial on understanding,
       creating and using regular expressions in Perl.  It serves
       as a complement to the reference page on regular
       expressions perlre.  Regular expressions are an integral
       part of the "m//", "s///", "qr//" and "split" operators
       and so this tutorial also overlaps with "Regexp Quote-Like
       Operators" in perlop and "split" in perlfunc.

       Perl is widely renowned for excellence in text processing,
       and regular expressions are one of the big factors behind
       this fame.  Perl regular expressions display an efficiency
       and flexibility unknown in most other computer languages.
       Mastering even the basics of regular expressions will
       allow you to manipulate text with surprising ease.

       What is a regular expression?  A regular expression is
       simply a string that describes a pattern.  Patterns are in
       common use these days; examples are the patterns typed
       into a search engine to find web pages and the patterns
       used to list files in a directory, e.g., "ls *.txt" or
       "dir *.*".  In Perl, the patterns described by regular
       expressions are used to search strings, extract desired
       parts of strings, and to do search and replace operations.

       Regular expressions have the undeserved reputation of
       being abstract and difficult to understand.  Regular
       expressions are constructed using simple concepts like
       conditionals and loops and are no more difficult to
       understand than the corresponding "if" conditionals and
       "while" loops in the Perl language itself.  In fact, the
       main challenge in learning regular expressions is just
       getting used to the terse notation used to express these
       concepts.

       This tutorial flattens the learning curve by discussing
       regular expression concepts, along with their notation,
       one at a time and with many examples.  The first part of
       the tutorial will progress from the simplest word searches
       to the basic regular expression concepts.  If you master
       the first part, you will have all the tools needed to
       solve about 98% of your needs.  The second part of the
       tutorial is for those comfortable with the basics and
       hungry for more power tools.  It discusses the more
       advanced regular expression operators and introduces the
       latest cutting edge innovations in 5.6.0.

       A note: to save time, 'regular expression' is often
       abbreviated as regexp or regex.  Regexp is a more natural
       abbreviation than regex, but is harder to pronounce.  The
       Perl pod documentation is evenly split on regexp vs regex;



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       in Perl, there is more than one way to abbreviate it.
       We'll use regexp in this tutorial.

Part 1: The basics
   Simple word matching
       The simplest regexp is simply a word, or more generally, a
       string of characters.  A regexp consisting of a word
       matches any string that contains that word:

           "Hello World" =~ /World/;  # matches

       What is this Perl statement all about? "Hello World" is a
       simple double quoted string.  "World" is the regular
       expression and the "//" enclosing "/World/" tells Perl to
       search a string for a match.  The operator "=~" associates
       the string with the regexp match and produces a true value
       if the regexp matched, or false if the regexp did not
       match.  In our case, "World" matches the second word in
       "Hello World", so the expression is true.  Expressions
       like this are useful in conditionals:

           if ("Hello World" =~ /World/) {
               print "It matches\n";
           }
           else {
               print "It doesn't match\n";
           }

       There are useful variations on this theme.  The sense of
       the match can be reversed by using the "!~" operator:

           if ("Hello World" !~ /World/) {
               print "It doesn't match\n";
           }
           else {
               print "It matches\n";
           }

       The literal string in the regexp can be replaced by a
       variable:

           $greeting = "World";
           if ("Hello World" =~ /$greeting/) {
               print "It matches\n";
           }
           else {
               print "It doesn't match\n";
           }

       If you're matching against the special default variable
       $_, the "$_ =~" part can be omitted:






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           $_ = "Hello World";
           if (/World/) {
               print "It matches\n";
           }
           else {
               print "It doesn't match\n";
           }

       And finally, the "//" default delimiters for a match can
       be changed to arbitrary delimiters by putting an 'm' out
       front:

           "Hello World" =~ m!World!;   # matches, delimited by '!'
           "Hello World" =~ m{World};   # matches, note the matching '{}'
           "/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
                                        # '/' becomes an ordinary char

       "/World/", "m!World!", and "m{World}" all represent the
       same thing.  When, e.g., the quote (""") is used as a
       delimiter, the forward slash '/' becomes an ordinary
       character and can be used in this regexp without trouble.

       Let's consider how different regexps would match "Hello
       World":

           "Hello World" =~ /world/;  # doesn't match
           "Hello World" =~ /o W/;    # matches
           "Hello World" =~ /oW/;     # doesn't match
           "Hello World" =~ /World /; # doesn't match

       The first regexp "world" doesn't match because regexps are
       case-sensitive.  The second regexp matches because the
       substring 'o W' occurs in the string "Hello World".  The
       space character ' ' is treated like any other character in
       a regexp and is needed to match in this case.  The lack of
       a space character is the reason the third regexp 'oW'
       doesn't match.  The fourth regexp 'World ' doesn't match
       because there is a space at the end of the regexp, but not
       at the end of the string.  The lesson here is that regexps
       must match a part of the string exactly in order for the
       statement to be true.

       If a regexp matches in more than one place in the string,
       Perl will always match at the earliest possible point in
       the string:

           "Hello World" =~ /o/;       # matches 'o' in 'Hello'
           "That hat is red" =~ /hat/; # matches 'hat' in 'That'

       With respect to character matching, there are a few more
       points you need to know about.   First of all, not all
       characters can be used 'as is' in a match.  Some
       characters, called metacharacters, are reserved for use in
       regexp notation.  The metacharacters are



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           {}[]()^$.|*+?\

       The significance of each of these will be explained in the
       rest of the tutorial, but for now, it is important only to
       know that a metacharacter can be matched by putting a
       backslash before it:

           "2+2=4" =~ /2+2/;    # doesn't match, + is a metacharacter
           "2+2=4" =~ /2\+2/;   # matches, \+ is treated like an ordinary +
           "The interval is [0,1)." =~ /[0,1)./     # is a syntax error!
           "The interval is [0,1)." =~ /\[0,1\)\./  # matches
           "#!/usr/bin/perl" =~ /#!\/usr\/bin\/perl/;  # matches

       In the last regexp, the forward slash '/' is also
       backslashed, because it is used to delimit the regexp.
       This can lead to LTS (leaning toothpick syndrome),
       however, and it is often more readable to change
       delimiters.

           "#!/usr/bin/perl" =~ m!#\!/usr/bin/perl!;  # easier to read

       The backslash character '\' is a metacharacter itself and
       needs to be backslashed:

           'C:\WIN32' =~ /C:\\WIN/;   # matches

       In addition to the metacharacters, there are some ASCII
       characters which don't have printable character
       equivalents and are instead represented by escape
       sequences.  Common examples are "\t" for a tab, "\n" for a
       newline, "\r" for a carriage return and "\a" for a bell.
       If your string is better thought of as a sequence of
       arbitrary bytes, the octal escape sequence, e.g., "\033",
       or hexadecimal escape sequence, e.g., "\x1B" may be a more
       natural representation for your bytes.  Here are some
       examples of escapes:

           "1000\t2000" =~ m(0\t2)   # matches
           "1000\n2000" =~ /0\n20/   # matches
           "1000\t2000" =~ /\000\t2/ # doesn't match, "0" ne "\000"
           "cat"        =~ /\143\x61\x74/ # matches, but a weird way to spell cat

       If you've been around Perl a while, all this talk of
       escape sequences may seem familiar.  Similar escape
       sequences are used in double-quoted strings and in fact
       the regexps in Perl are mostly treated as double-quoted
       strings.  This means that variables can be used in regexps
       as well.  Just like double-quoted strings, the values of
       the variables in the regexp will be substituted in before
       the regexp is evaluated for matching purposes.  So we
       have:






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           $foo = 'house';
           'housecat' =~ /$foo/;      # matches
           'cathouse' =~ /cat$foo/;   # matches
           'housecat' =~ /${foo}cat/; # matches

       So far, so good.  With the knowledge above you can already
       perform searches with just about any literal string regexp
       you can dream up.  Here is a very simple emulation of the
       Unix grep program:

           % cat > simple_grep
           #!/usr/bin/perl
           $regexp = shift;
           while (<>) {
               print if /$regexp/;
           }
           ^D

           % chmod +x simple_grep

           % simple_grep abba /usr/dict/words
           Babbage
           cabbage
           cabbages
           sabbath
           Sabbathize
           Sabbathizes
           sabbatical
           scabbard
           scabbards

       This program is easy to understand.  "#!/usr/bin/perl" is
       the standard way to invoke a perl program from the shell.
       "$regexp = shift;" saves the first command line argument
       as the regexp to be used, leaving the rest of the command
       line arguments to be treated as files.  "while (<>)" loops
       over all the lines in all the files.  For each line,
       "print if /$regexp/;" prints the line if the regexp
       matches the line.  In this line, both "print" and
       "/$regexp/" use the default variable $_ implicitly.

       With all of the regexps above, if the regexp matched
       anywhere in the string, it was considered a match.
       Sometimes, however, we'd like to specify where in the
       string the regexp should try to match.  To do this, we
       would use the anchor metacharacters "^" and "$".  The
       anchor "^" means match at the beginning of the string and
       the anchor "$" means match at the end of the string, or
       before a newline at the end of the string.  Here is how
       they are used:







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           "housekeeper" =~ /keeper/;    # matches
           "housekeeper" =~ /^keeper/;   # doesn't match
           "housekeeper" =~ /keeper$/;   # matches
           "housekeeper\n" =~ /keeper$/; # matches

       The second regexp doesn't match because "^" constrains
       "keeper" to match only at the beginning of the string, but
       "housekeeper" has keeper starting in the middle.  The
       third regexp does match, since the "$" constrains "keeper"
       to match only at the end of the string.

       When both "^" and "$" are used at the same time, the
       regexp has to match both the beginning and the end of the
       string, i.e., the regexp matches the whole string.
       Consider

           "keeper" =~ /^keep$/;      # doesn't match
           "keeper" =~ /^keeper$/;    # matches
           ""       =~ /^$/;          # ^$ matches an empty string

       The first regexp doesn't match because the string has more
       to it than "keep".  Since the second regexp is exactly the
       string, it matches.  Using both "^" and "$" in a regexp
       forces the complete string to match, so it gives you
       complete control over which strings match and which don't.
       Suppose you are looking for a fellow named bert, off in a
       string by himself:

           "dogbert" =~ /bert/;   # matches, but not what you want

           "dilbert" =~ /^bert/;  # doesn't match, but ..
           "bertram" =~ /^bert/;  # matches, so still not good enough

           "bertram" =~ /^bert$/; # doesn't match, good
           "dilbert" =~ /^bert$/; # doesn't match, good
           "bert"    =~ /^bert$/; # matches, perfect

       Of course, in the case of a literal string, one could just
       as easily use the string comparison "$string eq 'bert'"
       and it would be more efficient.   The  "^...$" regexp
       really becomes useful when we add in the more powerful
       regexp tools below.

   Using character classes
       Although one can already do quite a lot with the literal
       string regexps above, we've only scratched the surface of
       regular expression technology.  In this and subsequent
       sections we will introduce regexp concepts (and associated
       metacharacter notations) that will allow a regexp to not
       just represent a single character sequence, but a whole
       class of them.

       One such concept is that of a character class.  A
       character class allows a set of possible characters,



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       rather than just a single character, to match at a
       particular point in a regexp.  Character classes are
       denoted by brackets "[...]", with the set of characters to
       be possibly matched inside.  Here are some examples:

           /cat/;       # matches 'cat'
           /[bcr]at/;   # matches 'bat, 'cat', or 'rat'
           /item[0123456789]/;  # matches 'item0' or ... or 'item9'
           "abc" =~ /[cab]/;    # matches 'a'

       In the last statement, even though 'c' is the first
       character in the class, 'a' matches because the first
       character position in the string is the earliest point at
       which the regexp can match.

           /[yY][eE][sS]/;      # match 'yes' in a case-insensitive way
                                # 'yes', 'Yes', 'YES', etc.

       This regexp displays a common task: perform a case-
       insensitive match.  Perl provides a way of avoiding all
       those brackets by simply appending an 'i' to the end of
       the match.  Then "/[yY][eE][sS]/;" can be rewritten as
       "/yes/i;".  The 'i' stands for case-insensitive and is an
       example of a modifier of the matching operation.  We will
       meet other modifiers later in the tutorial.

       We saw in the section above that there were ordinary
       characters, which represented themselves, and special
       characters, which needed a backslash "\" to represent
       themselves.  The same is true in a character class, but
       the sets of ordinary and special characters inside a
       character class are different than those outside a
       character class.  The special characters for a character
       class are "-]\^$" (and the pattern delimiter, whatever it
       is).  "]" is special because it denotes the end of a
       character class.  "$" is special because it denotes a
       scalar variable.  "\" is special because it is used in
       escape sequences, just like above.  Here is how the
       special characters "]$\" are handled:

          /[\]c]def/; # matches ']def' or 'cdef'
          $x = 'bcr';
          /[$x]at/;   # matches 'bat', 'cat', or 'rat'
          /[\$x]at/;  # matches '$at' or 'xat'
          /[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat'

       The last two are a little tricky.  In "[\$x]", the
       backslash protects the dollar sign, so the character class
       has two members "$" and "x".  In "[\\$x]", the backslash
       is protected, so $x is treated as a variable and
       substituted in double quote fashion.

       The special character '-' acts as a range operator within
       character classes, so that a contiguous set of characters



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       can be written as a range.  With ranges, the unwieldy
       "[0123456789]" and "[abc...xyz]" become the svelte "[0-9]"
       and "[a-z]".  Some examples are

           /item[0-9]/;  # matches 'item0' or ... or 'item9'
           /[0-9bx-z]aa/;  # matches '0aa', ..., '9aa',
                           # 'baa', 'xaa', 'yaa', or 'zaa'
           /[0-9a-fA-F]/;  # matches a hexadecimal digit
           /[0-9a-zA-Z_]/; # matches a "word" character,
                           # like those in a Perl variable name

       If '-' is the first or last character in a character
       class, it is treated as an ordinary character; "[-ab]",
       "[ab-]" and "[a\-b]" are all equivalent.

       The special character "^" in the first position of a
       character class denotes a negated character class, which
       matches any character but those in the brackets.  Both
       "[...]" and "[^...]" must match a character, or the match
       fails.  Then

           /[^a]at/;  # doesn't match 'aat' or 'at', but matches
                      # all other 'bat', 'cat, '0at', '%at', etc.
           /[^0-9]/;  # matches a non-numeric character
           /[a^]at/;  # matches 'aat' or '^at'; here '^' is ordinary

       Now, even "[0-9]" can be a bother to write multiple times,
       so in the interest of saving keystrokes and making regexps
       more readable, Perl has several abbreviations for common
       character classes, as shown below.  Since the introduction
       of Unicode, these character classes match more than just a
       few characters in the ISO 8859-1 range.

       o   \d matches a digit, not just [0-9] but also digits
           from non-roman scripts

       o   \s matches a whitespace character, the set [\
           \t\r\n\f] and others

       o   \w matches a word character (alphanumeric or _), not
           just [0-9a-zA-Z_] but also digits and characters from
           non-roman scripts

       o   \D is a negated \d; it represents any other character
           than a digit, or [^\d]

       o   \S is a negated \s; it represents any non-whitespace
           character [^\s]

       o   \W is a negated \w; it represents any non-word
           character [^\w]

       o   The period '.' matches any character but "\n" (unless
           the modifier "//s" is in effect, as explained below).



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       The "\d\s\w\D\S\W" abbreviations can be used both inside
       and outside of character classes.  Here are some in use:

           /\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format
           /[\d\s]/;         # matches any digit or whitespace character
           /\w\W\w/;         # matches a word char, followed by a
                             # non-word char, followed by a word char
           /..rt/;           # matches any two chars, followed by 'rt'
           /end\./;          # matches 'end.'
           /end[.]/;         # same thing, matches 'end.'

       Because a period is a metacharacter, it needs to be
       escaped to match as an ordinary period. Because, for
       example, "\d" and "\w" are sets of characters, it is
       incorrect to think of "[^\d\w]" as "[\D\W]"; in fact
       "[^\d\w]" is the same as "[^\w]", which is the same as
       "[\W]". Think DeMorgan's laws.

       An anchor useful in basic regexps is the word anchor "\b".
       This matches a boundary between a word character and a
       non-word character "\w\W" or "\W\w":

           $x = "Housecat catenates house and cat";
           $x =~ /cat/;    # matches cat in 'housecat'
           $x =~ /\bcat/;  # matches cat in 'catenates'
           $x =~ /cat\b/;  # matches cat in 'housecat'
           $x =~ /\bcat\b/;  # matches 'cat' at end of string

       Note in the last example, the end of the string is
       considered a word boundary.

       You might wonder why '.' matches everything but "\n" - why
       not every character? The reason is that often one is
       matching against lines and would like to ignore the
       newline characters.  For instance, while the string "\n"
       represents one line, we would like to think of it as
       empty.  Then

           ""   =~ /^$/;    # matches
           "\n" =~ /^$/;    # matches, $ anchors before "\n"

           ""   =~ /./;      # doesn't match; it needs a char
           ""   =~ /^.$/;    # doesn't match; it needs a char
           "\n" =~ /^.$/;    # doesn't match; it needs a char other than "\n"
           "a"  =~ /^.$/;    # matches
           "a\n"  =~ /^.$/;  # matches, $ anchors before "\n"

       This behavior is convenient, because we usually want to
       ignore newlines when we count and match characters in a
       line.  Sometimes, however, we want to keep track of
       newlines.  We might even want "^" and "$" to anchor at the
       beginning and end of lines within the string, rather than
       just the beginning and end of the string.  Perl allows us
       to choose between ignoring and paying attention to



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       newlines by using the "//s" and "//m" modifiers.  "//s"
       and "//m" stand for single line and multi-line and they
       determine whether a string is to be treated as one
       continuous string, or as a set of lines.  The two
       modifiers affect two aspects of how the regexp is
       interpreted: 1) how the '.' character class is defined,
       and 2) where the anchors "^" and "$" are able to match.
       Here are the four possible combinations:

       o   no modifiers (//): Default behavior.  '.' matches any
           character except "\n".  "^" matches only at the
           beginning of the string and "$" matches only at the
           end or before a newline at the end.

       o   s modifier (//s): Treat string as a single long line.
           '.' matches any character, even "\n".  "^" matches
           only at the beginning of the string and "$" matches
           only at the end or before a newline at the end.

       o   m modifier (//m): Treat string as a set of multiple
           lines.  '.' matches any character except "\n".  "^"
           and "$" are able to match at the start or end of any
           line within the string.

       o   both s and m modifiers (//sm): Treat string as a
           single long line, but detect multiple lines.  '.'
           matches any character, even "\n".  "^" and "$",
           however, are able to match at the start or end of any
           line within the string.

       Here are examples of "//s" and "//m" in action:

           $x = "There once was a girl\nWho programmed in Perl\n";

           $x =~ /^Who/;   # doesn't match, "Who" not at start of string
           $x =~ /^Who/s;  # doesn't match, "Who" not at start of string
           $x =~ /^Who/m;  # matches, "Who" at start of second line
           $x =~ /^Who/sm; # matches, "Who" at start of second line

           $x =~ /girl.Who/;   # doesn't match, "." doesn't match "\n"
           $x =~ /girl.Who/s;  # matches, "." matches "\n"
           $x =~ /girl.Who/m;  # doesn't match, "." doesn't match "\n"
           $x =~ /girl.Who/sm; # matches, "." matches "\n"

       Most of the time, the default behavior is what is wanted,
       but "//s" and "//m" are occasionally very useful.  If
       "//m" is being used, the start of the string can still be
       matched with "\A" and the end of the string can still be
       matched with the anchors "\Z" (matches both the end and
       the newline before, like "$"), and "\z" (matches only the
       end):

           $x =~ /^Who/m;   # matches, "Who" at start of second line
           $x =~ /\AWho/m;  # doesn't match, "Who" is not at start of string



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           $x =~ /girl$/m;  # matches, "girl" at end of first line
           $x =~ /girl\Z/m; # doesn't match, "girl" is not at end of string

           $x =~ /Perl\Z/m; # matches, "Perl" is at newline before end
           $x =~ /Perl\z/m; # doesn't match, "Perl" is not at end of string

       We now know how to create choices among classes of
       characters in a regexp.  What about choices among words or
       character strings? Such choices are described in the next
       section.

   Matching this or that
       Sometimes we would like our regexp to be able to match
       different possible words or character strings.  This is
       accomplished by using the alternation metacharacter "|".
       To match "dog" or "cat", we form the regexp "dog|cat".  As
       before, Perl will try to match the regexp at the earliest
       possible point in the string.  At each character position,
       Perl will first try to match the first alternative, "dog".
       If "dog" doesn't match, Perl will then try the next
       alternative, "cat".  If "cat" doesn't match either, then
       the match fails and Perl moves to the next position in the
       string.  Some examples:

           "cats and dogs" =~ /cat|dog|bird/;  # matches "cat"
           "cats and dogs" =~ /dog|cat|bird/;  # matches "cat"

       Even though "dog" is the first alternative in the second
       regexp, "cat" is able to match earlier in the string.

           "cats"          =~ /c|ca|cat|cats/; # matches "c"
           "cats"          =~ /cats|cat|ca|c/; # matches "cats"

       Here, all the alternatives match at the first string
       position, so the first alternative is the one that
       matches.  If some of the alternatives are truncations of
       the others, put the longest ones first to give them a
       chance to match.

           "cab" =~ /a|b|c/ # matches "c"
                            # /a|b|c/ == /[abc]/

       The last example points out that character classes are
       like alternations of characters.  At a given character
       position, the first alternative that allows the regexp
       match to succeed will be the one that matches.

   Grouping things and hierarchical matching
       Alternation allows a regexp to choose among alternatives,
       but by itself it is unsatisfying.  The reason is that each
       alternative is a whole regexp, but sometime we want
       alternatives for just part of a regexp.  For instance,
       suppose we want to search for housecats or housekeepers.



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       The regexp "housecat|housekeeper" fits the bill, but is
       inefficient because we had to type "house" twice.  It
       would be nice to have parts of the regexp be constant,
       like "house", and some parts have alternatives, like
       "cat|keeper".

       The grouping metacharacters "()" solve this problem.
       Grouping allows parts of a regexp to be treated as a
       single unit.  Parts of a regexp are grouped by enclosing
       them in parentheses.  Thus we could solve the
       "housecat|housekeeper" by forming the regexp as
       "house(cat|keeper)".  The regexp "house(cat|keeper)" means
       match "house" followed by either "cat" or "keeper".  Some
       more examples are

           /(a|b)b/;    # matches 'ab' or 'bb'
           /(ac|b)b/;   # matches 'acb' or 'bb'
           /(^a|b)c/;   # matches 'ac' at start of string or 'bc' anywhere
           /(a|[bc])d/; # matches 'ad', 'bd', or 'cd'

           /house(cat|)/;  # matches either 'housecat' or 'house'
           /house(cat(s|)|)/;  # matches either 'housecats' or 'housecat' or
                               # 'house'.  Note groups can be nested.

           /(19|20|)\d\d/;  # match years 19xx, 20xx, or the Y2K problem, xx
           "20" =~ /(19|20|)\d\d/;  # matches the null alternative '()\d\d',
                                    # because '20\d\d' can't match

       Alternations behave the same way in groups as out of them:
       at a given string position, the leftmost alternative that
       allows the regexp to match is taken.  So in the last
       example at the first string position, "20" matches the
       second alternative, but there is nothing left over to
       match the next two digits "\d\d".  So Perl moves on to the
       next alternative, which is the null alternative and that
       works, since "20" is two digits.

       The process of trying one alternative, seeing if it
       matches, and moving on to the next alternative, while
       going back in the string from where the previous
       alternative was tried, if it doesn't, is called
       backtracking.  The term 'backtracking' comes from the idea
       that matching a regexp is like a walk in the woods.
       Successfully matching a regexp is like arriving at a
       destination.  There are many possible trailheads, one for
       each string position, and each one is tried in order, left
       to right.  From each trailhead there may be many paths,
       some of which get you there, and some which are dead ends.
       When you walk along a trail and hit a dead end, you have
       to backtrack along the trail to an earlier point to try
       another trail.  If you hit your destination, you stop
       immediately and forget about trying all the other trails.
       You are persistent, and only if you have tried all the
       trails from all the trailheads and not arrived at your



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       destination, do you declare failure.  To be concrete, here
       is a step-by-step analysis of what Perl does when it tries
       to match the regexp

           "abcde" =~ /(abd|abc)(df|d|de)/;

       0   Start with the first letter in the string 'a'.

       1   Try the first alternative in the first group 'abd'.

       2   Match 'a' followed by 'b'. So far so good.

       3   'd' in the regexp doesn't match 'c' in the string - a
           dead end.  So backtrack two characters and pick the
           second alternative in the first group 'abc'.

       4   Match 'a' followed by 'b' followed by 'c'.  We are on
           a roll and have satisfied the first group. Set $1 to
           'abc'.

       5   Move on to the second group and pick the first
           alternative 'df'.

       6   Match the 'd'.

       7   'f' in the regexp doesn't match 'e' in the string, so
           a dead end.  Backtrack one character and pick the
           second alternative in the second group 'd'.

       8   'd' matches. The second grouping is satisfied, so set
           $2 to 'd'.

       9   We are at the end of the regexp, so we are done! We
           have matched 'abcd' out of the string "abcde".

       There are a couple of things to note about this analysis.
       First, the third alternative in the second group 'de' also
       allows a match, but we stopped before we got to it - at a
       given character position, leftmost wins.  Second, we were
       able to get a match at the first character position of the
       string 'a'.  If there were no matches at the first
       position, Perl would move to the second character position
       'b' and attempt the match all over again.  Only when all
       possible paths at all possible character positions have
       been exhausted does Perl give up and declare
       "$string =~ /(abd|abc)(df|d|de)/;" to be false.

       Even with all this work, regexp matching happens
       remarkably fast.  To speed things up, Perl compiles the
       regexp into a compact sequence of opcodes that can often
       fit inside a processor cache.  When the code is executed,
       these opcodes can then run at full throttle and search
       very quickly.




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   Extracting matches
       The grouping metacharacters "()" also serve another
       completely different function: they allow the extraction
       of the parts of a string that matched.  This is very
       useful to find out what matched and for text processing in
       general.  For each grouping, the part that matched inside
       goes into the special variables $1, $2, etc.  They can be
       used just as ordinary variables:

           # extract hours, minutes, seconds
           if ($time =~ /(\d\d):(\d\d):(\d\d)/) {    # match hh:mm:ss format
               $hours = $1;
               $minutes = $2;
               $seconds = $3;
           }

       Now, we know that in scalar context,
       "$time =~ /(\d\d):(\d\d):(\d\d)/" returns a true or false
       value.  In list context, however, it returns the list of
       matched values "($1,$2,$3)".  So we could write the code
       more compactly as

           # extract hours, minutes, seconds
           ($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);

       If the groupings in a regexp are nested, $1 gets the group
       with the leftmost opening parenthesis, $2 the next opening
       parenthesis, etc.  Here is a regexp with nested groups:

           /(ab(cd|ef)((gi)|j))/;
            1  2      34

       If this regexp matches, $1 contains a string starting with
       'ab', $2 is either set to 'cd' or 'ef', $3 equals either
       'gi' or 'j', and $4 is either set to 'gi', just like $3,
       or it remains undefined.

       For convenience, Perl sets $+ to the string held by the
       highest numbered $1, $2,... that got assigned (and,
       somewhat related, $^N to the value of the $1, $2,... most-
       recently assigned; i.e. the $1, $2,... associated with the
       rightmost closing parenthesis used in the match).

   Backreferences
       Closely associated with the matching variables $1, $2, ...
       are the backreferences "\1", "\2",...  Backreferences are
       simply matching variables that can be used inside a
       regexp.  This is a really nice feature -- what matches
       later in a regexp is made to depend on what matched
       earlier in the regexp.  Suppose we wanted to look for
       doubled words in a text, like 'the the'.  The following
       regexp finds all 3-letter doubles with a space in between:

           /\b(\w\w\w)\s\1\b/;



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       The grouping assigns a value to \1, so that the same 3
       letter sequence is used for both parts.

       A similar task is to find words consisting of two
       identical parts:

           % simple_grep '^(\w\w\w\w|\w\w\w|\w\w|\w)\1$' /usr/dict/words
           beriberi
           booboo
           coco
           mama
           murmur
           papa

       The regexp has a single grouping which considers 4-letter
       combinations, then 3-letter combinations, etc., and uses
       "\1" to look for a repeat.  Although $1 and "\1" represent
       the same thing, care should be taken to use matched
       variables $1, $2,... only outside a regexp and
       backreferences "\1", "\2",... only inside a regexp; not
       doing so may lead to surprising and unsatisfactory
       results.

   Relative backreferences
       Counting the opening parentheses to get the correct number
       for a backreference is errorprone as soon as there is more
       than one capturing group.  A more convenient technique
       became available with Perl 5.10: relative backreferences.
       To refer to the immediately preceding capture group one
       now may write "\g{-1}", the next but last is available via
       "\g{-2}", and so on.

       Another good reason in addition to readability and
       maintainability for using relative backreferences  is
       illustrated by the following example, where a simple
       pattern for matching peculiar strings is used:

           $a99a = '([a-z])(\d)\2\1';   # matches a11a, g22g, x33x, etc.

       Now that we have this pattern stored as a handy string, we
       might feel tempted to use it as a part of some other
       pattern:

           $line = "code=e99e";
           if ($line =~ /^(\w+)=$a99a$/){   # unexpected behavior!
               print "$1 is valid\n";
           } else {
               print "bad line: '$line'\n";
           }

       But this doesn't match -- at least not the way one might
       expect. Only after inserting the interpolated $a99a and
       looking at the resulting full text of the regexp is it
       obvious that the backreferences have backfired -- the



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       subexpression "(\w+)" has snatched number 1 and demoted
       the groups in $a99a by one rank. This can be avoided by
       using relative backreferences:

           $a99a = '([a-z])(\d)\g{-1}\g{-2}';  # safe for being interpolated

   Named backreferences
       Perl 5.10 also introduced named capture buffers and named
       backreferences.  To attach a name to a capturing group,
       you write either "(?<name>...)" or "(?'name'...)".  The
       backreference may then be written as "\g{name}".  It is
       permissible to attach the same name to more than one
       group, but then only the leftmost one of the eponymous set
       can be referenced.  Outside of the pattern a named capture
       buffer is accessible through the "%+" hash.

       Assuming that we have to match calendar dates which may be
       given in one of the three formats yyyy-mm-dd, mm/dd/yyyy
       or dd.mm.yyyy, we can write three suitable patterns where
       we use 'd', 'm' and 'y' respectively as the names of the
       buffers capturing the pertaining components of a date. The
       matching operation combines the three patterns as
       alternatives:

           $fmt1 = '(?<y>\d\d\d\d)-(?<m>\d\d)-(?<d>\d\d)';
           $fmt2 = '(?<m>\d\d)/(?<d>\d\d)/(?<y>\d\d\d\d)';
           $fmt3 = '(?<d>\d\d)\.(?<m>\d\d)\.(?<y>\d\d\d\d)';
           for my $d qw( 2006-10-21 15.01.2007 10/31/2005 ){
               if ( $d =~ m{$fmt1|$fmt2|$fmt3} ){
                   print "day=$+{d} month=$+{m} year=$+{y}\n";
               }
           }

       If any of the alternatives matches, the hash "%+" is bound
       to contain the three key-value pairs.

   Alternative capture group numbering
       Yet another capturing group numbering technique (also as
       from Perl 5.10) deals with the problem of referring to
       groups within a set of alternatives.  Consider a pattern
       for matching a time of the day, civil or military style:

           if ( $time =~ /(\d\d|\d):(\d\d)|(\d\d)(\d\d)/ ){
               # process hour and minute
           }

       Processing the results requires an additional if statement
       to determine whether $1 and $2 or $3 and $4 contain the
       goodies. It would be easier if we could use buffer numbers
       1 and 2 in second alternative as well, and this is exactly
       what the parenthesized construct "(?|...)", set around an
       alternative achieves. Here is an extended version of the
       previous pattern:




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           if ( $time =~ /(?|(\d\d|\d):(\d\d)|(\d\d)(\d\d))\s+([A-Z][A-Z][A-Z])/ ){
               print "hour=$1 minute=$2 zone=$3\n";
           }

       Within the alternative numbering group, buffer numbers
       start at the same position for each alternative. After the
       group, numbering continues with one higher than the
       maximum reached across all the alternatives.

   Position information
       In addition to what was matched, Perl (since 5.6.0) also
       provides the positions of what was matched as contents of
       the "@-" and "@+" arrays. "$-[0]" is the position of the
       start of the entire match and $+[0] is the position of the
       end. Similarly, "$-[n]" is the position of the start of
       the $n match and $+[n] is the position of the end. If $n
       is undefined, so are "$-[n]" and $+[n]. Then this code

           $x = "Mmm...donut, thought Homer";
           $x =~ /^(Mmm|Yech)\.\.\.(donut|peas)/; # matches
           foreach $expr (1..$#-) {
               print "Match $expr: '${$expr}' at position ($-[$expr],$+[$expr])\n";
           }

       prints

           Match 1: 'Mmm' at position (0,3)
           Match 2: 'donut' at position (6,11)

       Even if there are no groupings in a regexp, it is still
       possible to find out what exactly matched in a string.  If
       you use them, Perl will set "$`" to the part of the string
       before the match, will set $& to the part of the string
       that matched, and will set "$'" to the part of the string
       after the match.  An example:

           $x = "the cat caught the mouse";
           $x =~ /cat/;  # $` = 'the ', $& = 'cat', $' = ' caught the mouse'
           $x =~ /the/;  # $` = '', $& = 'the', $' = ' cat caught the mouse'

       In the second match, "$`" equals '' because the regexp
       matched at the first character position in the string and
       stopped; it never saw the second 'the'.  It is important
       to note that using "$`" and "$'" slows down regexp
       matching quite a bit, while $& slows it down to a lesser
       extent, because if they are used in one regexp in a
       program, they are generated for all regexps in the
       program.  So if raw performance is a goal of your
       application, they should be avoided.  If you need to
       extract the corresponding substrings, use "@-" and "@+"
       instead:






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           $` is the same as substr( $x, 0, $-[0] )
           $& is the same as substr( $x, $-[0], $+[0]-$-[0] )
           $' is the same as substr( $x, $+[0] )

   Non-capturing groupings
       A group that is required to bundle a set of alternatives
       may or may not be useful as a capturing group.  If it
       isn't, it just creates a superfluous addition to the set
       of available capture buffer values, inside as well as
       outside the regexp.  Non-capturing groupings, denoted by
       "(?:regexp)", still allow the regexp to be treated as a
       single unit, but don't establish a capturing buffer at the
       same time.  Both capturing and non-capturing groupings are
       allowed to co-exist in the same regexp.  Because there is
       no extraction, non-capturing groupings are faster than
       capturing groupings.  Non-capturing groupings are also
       handy for choosing exactly which parts of a regexp are to
       be extracted to matching variables:

           # match a number, $1-$4 are set, but we only want $1
           /([+-]?\ *(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?)/;

           # match a number faster , only $1 is set
           /([+-]?\ *(?:\d+(?:\.\d*)?|\.\d+)(?:[eE][+-]?\d+)?)/;

           # match a number, get $1 = whole number, $2 = exponent
           /([+-]?\ *(?:\d+(?:\.\d*)?|\.\d+)(?:[eE]([+-]?\d+))?)/;

       Non-capturing groupings are also useful for removing
       nuisance elements gathered from a split operation where
       parentheses are required for some reason:

           $x = '12aba34ba5';
           @num = split /(a|b)+/, $x;    # @num = ('12','a','34','b','5')
           @num = split /(?:a|b)+/, $x;  # @num = ('12','34','5')

   Matching repetitions
       The examples in the previous section display an annoying
       weakness.  We were only matching 3-letter words, or chunks
       of words of 4 letters or less.  We'd like to be able to
       match words or, more generally, strings of any length,
       without writing out tedious alternatives like
       "\w\w\w\w|\w\w\w|\w\w|\w".

       This is exactly the problem the quantifier metacharacters
       "?", "*", "+", and "{}" were created for.  They allow us
       to delimit the number of repeats for a portion of a regexp
       we consider to be a match.  Quantifiers are put
       immediately after the character, character class, or
       grouping that we want to specify.  They have the following
       meanings:

       o   "a?" means: match 'a' 1 or 0 times




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       o   "a*" means: match 'a' 0 or more times, i.e., any
           number of times

       o   "a+" means: match 'a' 1 or more times, i.e., at least
           once

       o   "a{n,m}" means: match at least "n" times, but not more
           than "m" times.

       o   "a{n,}" means: match at least "n" or more times

       o   "a{n}" means: match exactly "n" times

       Here are some examples:

           /[a-z]+\s+\d*/;  # match a lowercase word, at least one space, and
                            # any number of digits
           /(\w+)\s+\1/;    # match doubled words of arbitrary length
           /y(es)?/i;       # matches 'y', 'Y', or a case-insensitive 'yes'
           $year =~ /\d{2,4}/;  # make sure year is at least 2 but not more
                                # than 4 digits
           $year =~ /\d{4}|\d{2}/;    # better match; throw out 3 digit dates
           $year =~ /\d{2}(\d{2})?/;  # same thing written differently. However,
                                      # this produces $1 and the other does not.

           % simple_grep '^(\w+)\1$' /usr/dict/words   # isn't this easier?
           beriberi
           booboo
           coco
           mama
           murmur
           papa

       For all of these quantifiers, Perl will try to match as
       much of the string as possible, while still allowing the
       regexp to succeed.  Thus with "/a?.../", Perl will first
       try to match the regexp with the "a" present; if that
       fails, Perl will try to match the regexp without the "a"
       present.  For the quantifier "*", we get the following:

           $x = "the cat in the hat";
           $x =~ /^(.*)(cat)(.*)$/; # matches,
                                    # $1 = 'the '
                                    # $2 = 'cat'
                                    # $3 = ' in the hat'

       Which is what we might expect, the match finds the only
       "cat" in the string and locks onto it.  Consider, however,
       this regexp:

           $x =~ /^(.*)(at)(.*)$/; # matches,
                                   # $1 = 'the cat in the h'
                                   # $2 = 'at'
                                   # $3 = ''   (0 characters match)



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       One might initially guess that Perl would find the "at" in
       "cat" and stop there, but that wouldn't give the longest
       possible string to the first quantifier ".*".  Instead,
       the first quantifier ".*" grabs as much of the string as
       possible while still having the regexp match.  In this
       example, that means having the "at" sequence with the
       final "at" in the string.  The other important principle
       illustrated here is that when there are two or more
       elements in a regexp, the leftmost quantifier, if there is
       one, gets to grab as much the string as possible, leaving
       the rest of the regexp to fight over scraps.  Thus in our
       example, the first quantifier ".*" grabs most of the
       string, while the second quantifier ".*" gets the empty
       string.   Quantifiers that grab as much of the string as
       possible are called maximal match or greedy quantifiers.

       When a regexp can match a string in several different
       ways, we can use the principles above to predict which way
       the regexp will match:

       o   Principle 0: Taken as a whole, any regexp will be
           matched at the earliest possible position in the
           string.

       o   Principle 1: In an alternation "a|b|c...", the
           leftmost alternative that allows a match for the whole
           regexp will be the one used.

       o   Principle 2: The maximal matching quantifiers "?",
           "*", "+" and "{n,m}" will in general match as much of
           the string as possible while still allowing the whole
           regexp to match.

       o   Principle 3: If there are two or more elements in a
           regexp, the leftmost greedy quantifier, if any, will
           match as much of the string as possible while still
           allowing the whole regexp to match.  The next leftmost
           greedy quantifier, if any, will try to match as much
           of the string remaining available to it as possible,
           while still allowing the whole regexp to match.  And
           so on, until all the regexp elements are satisfied.

       As we have seen above, Principle 0 overrides the others --
       the regexp will be matched as early as possible, with the
       other principles determining how the regexp matches at
       that earliest character position.

       Here is an example of these principles in action:

           $x = "The programming republic of Perl";
           $x =~ /^(.+)(e|r)(.*)$/;  # matches,
                                     # $1 = 'The programming republic of Pe'
                                     # $2 = 'r'
                                     # $3 = 'l'



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       This regexp matches at the earliest string position, 'T'.
       One might think that "e", being leftmost in the
       alternation, would be matched, but "r" produces the
       longest string in the first quantifier.

           $x =~ /(m{1,2})(.*)$/;  # matches,
                                   # $1 = 'mm'
                                   # $2 = 'ing republic of Perl'

       Here, The earliest possible match is at the first 'm' in
       "programming". "m{1,2}" is the first quantifier, so it
       gets to match a maximal "mm".

           $x =~ /.*(m{1,2})(.*)$/;  # matches,
                                     # $1 = 'm'
                                     # $2 = 'ing republic of Perl'

       Here, the regexp matches at the start of the string. The
       first quantifier ".*" grabs as much as possible, leaving
       just a single 'm' for the second quantifier "m{1,2}".

           $x =~ /(.?)(m{1,2})(.*)$/;  # matches,
                                       # $1 = 'a'
                                       # $2 = 'mm'
                                       # $3 = 'ing republic of Perl'

       Here, ".?" eats its maximal one character at the earliest
       possible position in the string, 'a' in "programming",
       leaving "m{1,2}" the opportunity to match both "m"'s.
       Finally,

           "aXXXb" =~ /(X*)/; # matches with $1 = ''

       because it can match zero copies of 'X' at the beginning
       of the string.  If you definitely want to match at least
       one 'X', use "X+", not "X*".

       Sometimes greed is not good.  At times, we would like
       quantifiers to match a minimal piece of string, rather
       than a maximal piece.  For this purpose, Larry Wall
       created the minimal match or non-greedy quantifiers "??",
       "*?", "+?", and "{}?".  These are the usual quantifiers
       with a "?" appended to them.  They have the following
       meanings:

       o   "a??" means: match 'a' 0 or 1 times. Try 0 first, then
           1.

       o   "a*?" means: match 'a' 0 or more times, i.e., any
           number of times, but as few times as possible

       o   "a+?" means: match 'a' 1 or more times, i.e., at least
           once, but as few times as possible




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       o   "a{n,m}?" means: match at least "n" times, not more
           than "m" times, as few times as possible

       o   "a{n,}?" means: match at least "n" times, but as few
           times as possible

       o   "a{n}?" means: match exactly "n" times.  Because we
           match exactly "n" times, "a{n}?" is equivalent to
           "a{n}" and is just there for notational consistency.

       Let's look at the example above, but with minimal
       quantifiers:

           $x = "The programming republic of Perl";
           $x =~ /^(.+?)(e|r)(.*)$/; # matches,
                                     # $1 = 'Th'
                                     # $2 = 'e'
                                     # $3 = ' programming republic of Perl'

       The minimal string that will allow both the start of the
       string "^" and the alternation to match is "Th", with the
       alternation "e|r" matching "e".  The second quantifier
       ".*" is free to gobble up the rest of the string.

           $x =~ /(m{1,2}?)(.*?)$/;  # matches,
                                     # $1 = 'm'
                                     # $2 = 'ming republic of Perl'

       The first string position that this regexp can match is at
       the first 'm' in "programming". At this position, the
       minimal "m{1,2}?"  matches just one 'm'.  Although the
       second quantifier ".*?" would prefer to match no
       characters, it is constrained by the end-of-string anchor
       "$" to match the rest of the string.

           $x =~ /(.*?)(m{1,2}?)(.*)$/;  # matches,
                                         # $1 = 'The progra'
                                         # $2 = 'm'
                                         # $3 = 'ming republic of Perl'

       In this regexp, you might expect the first minimal
       quantifier ".*?"  to match the empty string, because it is
       not constrained by a "^" anchor to match the beginning of
       the word.  Principle 0 applies here, however.  Because it
       is possible for the whole regexp to match at the start of
       the string, it will match at the start of the string.
       Thus the first quantifier has to match everything up to
       the first "m".  The second minimal quantifier matches just
       one "m" and the third quantifier matches the rest of the
       string.







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           $x =~ /(.??)(m{1,2})(.*)$/;  # matches,
                                        # $1 = 'a'
                                        # $2 = 'mm'
                                        # $3 = 'ing republic of Perl'

       Just as in the previous regexp, the first quantifier ".??"
       can match earliest at position 'a', so it does.  The
       second quantifier is greedy, so it matches "mm", and the
       third matches the rest of the string.

       We can modify principle 3 above to take into account non-
       greedy quantifiers:

       o   Principle 3: If there are two or more elements in a
           regexp, the leftmost greedy (non-greedy) quantifier,
           if any, will match as much (little) of the string as
           possible while still allowing the whole regexp to
           match.  The next leftmost greedy (non-greedy)
           quantifier, if any, will try to match as much (little)
           of the string remaining available to it as possible,
           while still allowing the whole regexp to match.  And
           so on, until all the regexp elements are satisfied.

       Just like alternation, quantifiers are also susceptible to
       backtracking.  Here is a step-by-step analysis of the
       example

           $x = "the cat in the hat";
           $x =~ /^(.*)(at)(.*)$/; # matches,
                                   # $1 = 'the cat in the h'
                                   # $2 = 'at'
                                   # $3 = ''   (0 matches)

       0   Start with the first letter in the string 't'.

       1   The first quantifier '.*' starts out by matching the
           whole string 'the cat in the hat'.

       2   'a' in the regexp element 'at' doesn't match the end
           of the string.  Backtrack one character.

       3   'a' in the regexp element 'at' still doesn't match the
           last letter of the string 't', so backtrack one more
           character.

       4   Now we can match the 'a' and the 't'.

       5   Move on to the third element '.*'.  Since we are at
           the end of the string and '.*' can match 0 times,
           assign it the empty string.

       6   We are done!

       Most of the time, all this moving forward and backtracking



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       happens quickly and searching is fast. There are some
       pathological regexps, however, whose execution time
       exponentially grows with the size of the string.  A
       typical structure that blows up in your face is of the
       form

           /(a|b+)*/;

       The problem is the nested indeterminate quantifiers.
       There are many different ways of partitioning a string of
       length n between the "+" and "*": one repetition with "b+"
       of length n, two repetitions with the first "b+" length k
       and the second with length n-k, m repetitions whose bits
       add up to length n, etc.  In fact there are an exponential
       number of ways to partition a string as a function of its
       length.  A regexp may get lucky and match early in the
       process, but if there is no match, Perl will try every
       possibility before giving up.  So be careful with nested
       "*"'s, "{n,m}"'s, and "+"'s.  The book Mastering Regular
       Expressions by Jeffrey Friedl gives a wonderful discussion
       of this and other efficiency issues.

   Possessive quantifiers
       Backtracking during the relentless search for a match may
       be a waste of time, particularly when the match is bound
       to fail.  Consider the simple pattern

           /^\w+\s+\w+$/; # a word, spaces, a word

       Whenever this is applied to a string which doesn't quite
       meet the pattern's expectations such as "abc  " or
       "abc  def ", the regex engine will backtrack,
       approximately once for each character in the string.  But
       we know that there is no way around taking all of the
       initial word characters to match the first repetition,
       that all spaces must be eaten by the middle part, and the
       same goes for the second word.

       With the introduction of the possessive quantifiers in
       Perl 5.10, we have a way of instructing the regex engine
       not to backtrack, with the usual quantifiers with a "+"
       appended to them.  This makes them greedy as well as
       stingy; once they succeed they won't give anything back to
       permit another solution. They have the following meanings:

       o   "a{n,m}+" means: match at least "n" times, not more
           than "m" times, as many times as possible, and don't
           give anything up. "a?+" is short for "a{0,1}+"

       o   "a{n,}+" means: match at least "n" times, but as many
           times as possible, and don't give anything up. "a*+"
           is short for "a{0,}+" and "a++" is short for "a{1,}+".

       o   "a{n}+" means: match exactly "n" times.  It is just



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           there for notational consistency.

       These possessive quantifiers represent a special case of a
       more general concept, the independent subexpression, see
       below.

       As an example where a possessive quantifier is suitable we
       consider matching a quoted string, as it appears in
       several programming languages.  The backslash is used as
       an escape character that indicates that the next character
       is to be taken literally, as another character for the
       string.  Therefore, after the opening quote, we expect a
       (possibly empty) sequence of alternatives: either some
       character except an unescaped quote or backslash or an
       escaped character.

           /"(?:[^"\\]++|\\.)*+"/;

   Building a regexp
       At this point, we have all the basic regexp concepts
       covered, so let's give a more involved example of a
       regular expression.  We will build a regexp that matches
       numbers.

       The first task in building a regexp is to decide what we
       want to match and what we want to exclude.  In our case,
       we want to match both integers and floating point numbers
       and we want to reject any string that isn't a number.

       The next task is to break the problem down into smaller
       problems that are easily converted into a regexp.

       The simplest case is integers.  These consist of a
       sequence of digits, with an optional sign in front.  The
       digits we can represent with "\d+" and the sign can be
       matched with "[+-]".  Thus the integer regexp is

           /[+-]?\d+/;  # matches integers

       A floating point number potentially has a sign, an
       integral part, a decimal point, a fractional part, and an
       exponent.  One or more of these parts is optional, so we
       need to check out the different possibilities.  Floating
       point numbers which are in proper form include 123.,
       0.345, .34, -1e6, and 25.4E-72.  As with integers, the
       sign out front is completely optional and can be matched
       by "[+-]?".  We can see that if there is no exponent,
       floating point numbers must have a decimal point,
       otherwise they are integers.  We might be tempted to model
       these with "\d*\.\d*", but this would also match just a
       single decimal point, which is not a number.  So the three
       cases of floating point number without exponent are





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          /[+-]?\d+\./;  # 1., 321., etc.
          /[+-]?\.\d+/;  # .1, .234, etc.
          /[+-]?\d+\.\d+/;  # 1.0, 30.56, etc.

       These can be combined into a single regexp with a three-
       way alternation:

          /[+-]?(\d+\.\d+|\d+\.|\.\d+)/;  # floating point, no exponent

       In this alternation, it is important to put '\d+\.\d+'
       before '\d+\.'.  If '\d+\.' were first, the regexp would
       happily match that and ignore the fractional part of the
       number.

       Now consider floating point numbers with exponents.  The
       key observation here is that both integers and numbers
       with decimal points are allowed in front of an exponent.
       Then exponents, like the overall sign, are independent of
       whether we are matching numbers with or without decimal
       points, and can be 'decoupled' from the mantissa.  The
       overall form of the regexp now becomes clear:

           /^(optional sign)(integer | f.p. mantissa)(optional exponent)$/;

       The exponent is an "e" or "E", followed by an integer.  So
       the exponent regexp is

          /[eE][+-]?\d+/;  # exponent

       Putting all the parts together, we get a regexp that
       matches numbers:

          /^[+-]?(\d+\.\d+|\d+\.|\.\d+|\d+)([eE][+-]?\d+)?$/;  # Ta da!

       Long regexps like this may impress your friends, but can
       be hard to decipher.  In complex situations like this, the
       "//x" modifier for a match is invaluable.  It allows one
       to put nearly arbitrary whitespace and comments into a
       regexp without affecting their meaning.  Using it, we can
       rewrite our 'extended' regexp in the more pleasing form

          /^
             [+-]?         # first, match an optional sign
             (             # then match integers or f.p. mantissas:
                 \d+\.\d+  # mantissa of the form a.b
                |\d+\.     # mantissa of the form a.
                |\.\d+     # mantissa of the form .b
                |\d+       # integer of the form a
             )
             ([eE][+-]?\d+)?  # finally, optionally match an exponent
          $/x;

       If whitespace is mostly irrelevant, how does one include
       space characters in an extended regexp? The answer is to



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       backslash it '\ ' or put it in a character class "[ ]".
       The same thing goes for pound signs, use "\#" or "[#]".
       For instance, Perl allows a space between the sign and the
       mantissa or integer, and we could add this to our regexp
       as follows:

          /^
             [+-]?\ *      # first, match an optional sign *and space*
             (             # then match integers or f.p. mantissas:
                 \d+\.\d+  # mantissa of the form a.b
                |\d+\.     # mantissa of the form a.
                |\.\d+     # mantissa of the form .b
                |\d+       # integer of the form a
             )
             ([eE][+-]?\d+)?  # finally, optionally match an exponent
          $/x;

       In this form, it is easier to see a way to simplify the
       alternation.  Alternatives 1, 2, and 4 all start with
       "\d+", so it could be factored out:

          /^
             [+-]?\ *      # first, match an optional sign
             (             # then match integers or f.p. mantissas:
                 \d+       # start out with a ...
                 (
                     \.\d* # mantissa of the form a.b or a.
                 )?        # ? takes care of integers of the form a
                |\.\d+     # mantissa of the form .b
             )
             ([eE][+-]?\d+)?  # finally, optionally match an exponent
          $/x;

       or written in the compact form,

           /^[+-]?\ *(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?$/;

       This is our final regexp.  To recap, we built a regexp by

       o   specifying the task in detail,

       o   breaking down the problem into smaller parts,

       o   translating the small parts into regexps,

       o   combining the regexps,

       o   and optimizing the final combined regexp.

       These are also the typical steps involved in writing a
       computer program.  This makes perfect sense, because
       regular expressions are essentially programs written in a
       little computer language that specifies patterns.




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   Using regular expressions in Perl
       The last topic of Part 1 briefly covers how regexps are
       used in Perl programs.  Where do they fit into Perl
       syntax?

       We have already introduced the matching operator in its
       default "/regexp/" and arbitrary delimiter "m!regexp!"
       forms.  We have used the binding operator "=~" and its
       negation "!~" to test for string matches.  Associated with
       the matching operator, we have discussed the single line
       "//s", multi-line "//m", case-insensitive "//i" and
       extended "//x" modifiers.  There are a few more things you
       might want to know about matching operators.

       Optimizing pattern evaluation

       We pointed out earlier that variables in regexps are
       substituted before the regexp is evaluated:

           $pattern = 'Seuss';
           while (<>) {
               print if /$pattern/;
           }

       This will print any lines containing the word "Seuss".  It
       is not as efficient as it could be, however, because Perl
       has to re-evaluate (or compile) $pattern each time through
       the loop.  If $pattern won't be changing over the lifetime
       of the script, we can add the "//o" modifier, which
       directs Perl to only perform variable substitutions once:

           #!/usr/bin/perl
           #    Improved simple_grep
           $regexp = shift;
           while (<>) {
               print if /$regexp/o;  # a good deal faster
           }

       Prohibiting substitution

       If you change $pattern after the first substitution
       happens, Perl will ignore it.  If you don't want any
       substitutions at all, use the special delimiter "m''":

           @pattern = ('Seuss');
           while (<>) {
               print if m'@pattern';  # matches literal '@pattern', not 'Seuss'
           }

       Similar to strings, "m''" acts like apostrophes on a
       regexp; all other "m" delimiters act like quotes.  If the
       regexp evaluates to the empty string, the regexp in the
       last successful match is used instead.  So we have




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           "dog" =~ /d/;  # 'd' matches
           "dogbert =~ //;  # this matches the 'd' regexp used before

       Global matching

       The final two modifiers "//g" and "//c" concern multiple
       matches.  The modifier "//g" stands for global matching
       and allows the matching operator to match within a string
       as many times as possible.  In scalar context, successive
       invocations against a string will have `"//g" jump from
       match to match, keeping track of position in the string as
       it goes along.  You can get or set the position with the
       "pos()" function.

       The use of "//g" is shown in the following example.
       Suppose we have a string that consists of words separated
       by spaces.  If we know how many words there are in
       advance, we could extract the words using groupings:

           $x = "cat dog house"; # 3 words
           $x =~ /^\s*(\w+)\s+(\w+)\s+(\w+)\s*$/; # matches,
                                                  # $1 = 'cat'
                                                  # $2 = 'dog'
                                                  # $3 = 'house'

       But what if we had an indeterminate number of words? This
       is the sort of task "//g" was made for.  To extract all
       words, form the simple regexp "(\w+)" and loop over all
       matches with "/(\w+)/g":

           while ($x =~ /(\w+)/g) {
               print "Word is $1, ends at position ", pos $x, "\n";
           }

       prints

           Word is cat, ends at position 3
           Word is dog, ends at position 7
           Word is house, ends at position 13

       A failed match or changing the target string resets the
       position.  If you don't want the position reset after
       failure to match, add the "//c", as in "/regexp/gc".  The
       current position in the string is associated with the
       string, not the regexp.  This means that different strings
       have different positions and their respective positions
       can be set or read independently.

       In list context, "//g" returns a list of matched
       groupings, or if there are no groupings, a list of matches
       to the whole regexp.  So if we wanted just the words, we
       could use





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           @words = ($x =~ /(\w+)/g);  # matches,
                                       # $word[0] = 'cat'
                                       # $word[1] = 'dog'
                                       # $word[2] = 'house'

       Closely associated with the "//g" modifier is the "\G"
       anchor.  The "\G" anchor matches at the point where the
       previous "//g" match left off.  "\G" allows us to easily
       do context-sensitive matching:

           $metric = 1;  # use metric units
           ...
           $x = <FILE>;  # read in measurement
           $x =~ /^([+-]?\d+)\s*/g;  # get magnitude
           $weight = $1;
           if ($metric) { # error checking
               print "Units error!" unless $x =~ /\Gkg\./g;
           }
           else {
               print "Units error!" unless $x =~ /\Glbs\./g;
           }
           $x =~ /\G\s+(widget|sprocket)/g;  # continue processing

       The combination of "//g" and "\G" allows us to process the
       string a bit at a time and use arbitrary Perl logic to
       decide what to do next.  Currently, the "\G" anchor is
       only fully supported when used to anchor to the start of
       the pattern.

       "\G" is also invaluable in processing fixed length records
       with regexps.  Suppose we have a snippet of coding region
       DNA, encoded as base pair letters "ATCGTTGAAT..." and we
       want to find all the stop codons "TGA".  In a coding
       region, codons are 3-letter sequences, so we can think of
       the DNA snippet as a sequence of 3-letter records.  The
       naive regexp

           # expanded, this is "ATC GTT GAA TGC AAA TGA CAT GAC"
           $dna = "ATCGTTGAATGCAAATGACATGAC";
           $dna =~ /TGA/;

       doesn't work; it may match a "TGA", but there is no
       guarantee that the match is aligned with codon boundaries,
       e.g., the substring "GTT GAA" gives a match.  A better
       solution is

           while ($dna =~ /(\w\w\w)*?TGA/g) {  # note the minimal *?
               print "Got a TGA stop codon at position ", pos $dna, "\n";
           }

       which prints

           Got a TGA stop codon at position 18
           Got a TGA stop codon at position 23



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       Position 18 is good, but position 23 is bogus.  What
       happened?

       The answer is that our regexp works well until we get past
       the last real match.  Then the regexp will fail to match a
       synchronized "TGA" and start stepping ahead one character
       position at a time, not what we want.  The solution is to
       use "\G" to anchor the match to the codon alignment:

           while ($dna =~ /\G(\w\w\w)*?TGA/g) {
               print "Got a TGA stop codon at position ", pos $dna, "\n";
           }

       This prints

           Got a TGA stop codon at position 18

       which is the correct answer.  This example illustrates
       that it is important not only to match what is desired,
       but to reject what is not desired.

       Search and replace

       Regular expressions also play a big role in search and
       replace operations in Perl.  Search and replace is
       accomplished with the "s///" operator.  The general form
       is "s/regexp/replacement/modifiers", with everything we
       know about regexps and modifiers applying in this case as
       well.  The "replacement" is a Perl double quoted string
       that replaces in the string whatever is matched with the
       "regexp".  The operator "=~" is also used here to
       associate a string with "s///".  If matching against $_,
       the "$_ =~" can be dropped.  If there is a match, "s///"
       returns the number of substitutions made, otherwise it
       returns false.  Here are a few examples:

           $x = "Time to feed the cat!";
           $x =~ s/cat/hacker/;   # $x contains "Time to feed the hacker!"
           if ($x =~ s/^(Time.*hacker)!$/$1 now!/) {
               $more_insistent = 1;
           }
           $y = "'quoted words'";
           $y =~ s/^'(.*)'$/$1/;  # strip single quotes,
                                  # $y contains "quoted words"

       In the last example, the whole string was matched, but
       only the part inside the single quotes was grouped.  With
       the "s///" operator, the matched variables $1, $2, etc.
       are immediately available for use in the replacement
       expression, so we use $1 to replace the quoted string with
       just what was quoted.  With the global modifier, "s///g"
       will search and replace all occurrences of the regexp in
       the string:




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           $x = "I batted 4 for 4";
           $x =~ s/4/four/;   # doesn't do it all:
                              # $x contains "I batted four for 4"
           $x = "I batted 4 for 4";
           $x =~ s/4/four/g;  # does it all:
                              # $x contains "I batted four for four"

       If you prefer 'regex' over 'regexp' in this tutorial, you
       could use the following program to replace it:

           % cat > simple_replace
           #!/usr/bin/perl
           $regexp = shift;
           $replacement = shift;
           while (<>) {
               s/$regexp/$replacement/go;
               print;
           }
           ^D

           % simple_replace regexp regex perlretut.pod

       In "simple_replace" we used the "s///g" modifier to
       replace all occurrences of the regexp on each line and the
       "s///o" modifier to compile the regexp only once.  As with
       "simple_grep", both the "print" and the
       "s/$regexp/$replacement/go" use $_ implicitly.

       A modifier available specifically to search and replace is
       the "s///e" evaluation modifier.  "s///e" wraps an
       "eval{...}" around the replacement string and the
       evaluated result is substituted for the matched substring.
       "s///e" is useful if you need to do a bit of computation
       in the process of replacing text.  This example counts
       character frequencies in a line:

           $x = "Bill the cat";
           $x =~ s/(.)/$chars{$1}++;$1/eg;  # final $1 replaces char with itself
           print "frequency of '$_' is $chars{$_}\n"
               foreach (sort {$chars{$b} <=> $chars{$a}} keys %chars);

       This prints

           frequency of ' ' is 2
           frequency of 't' is 2
           frequency of 'l' is 2
           frequency of 'B' is 1
           frequency of 'c' is 1
           frequency of 'e' is 1
           frequency of 'h' is 1
           frequency of 'i' is 1
           frequency of 'a' is 1

       As with the match "m//" operator, "s///" can use other



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       delimiters, such as "s!!!" and "s{}{}", and even "s{}//".
       If single quotes are used "s'''", then the regexp and
       replacement are treated as single quoted strings and there
       are no substitutions.  "s///" in list context returns the
       same thing as in scalar context, i.e., the number of
       matches.

       The split function

       The "split()" function is another place where a regexp is
       used.  "split /regexp/, string, limit" separates the
       "string" operand into a list of substrings and returns
       that list.  The regexp must be designed to match whatever
       constitutes the separators for the desired substrings.
       The "limit", if present, constrains splitting into no more
       than "limit" number of strings.  For example, to split a
       string into words, use

           $x = "Calvin and Hobbes";
           @words = split /\s+/, $x;  # $word[0] = 'Calvin'
                                      # $word[1] = 'and'
                                      # $word[2] = 'Hobbes'

       If the empty regexp "//" is used, the regexp always
       matches and the string is split into individual
       characters.  If the regexp has groupings, then the
       resulting list contains the matched substrings from the
       groupings as well.  For instance,

           $x = "/usr/bin/perl";
           @dirs = split m!/!, $x;  # $dirs[0] = ''
                                    # $dirs[1] = 'usr'
                                    # $dirs[2] = 'bin'
                                    # $dirs[3] = 'perl'
           @parts = split m!(/)!, $x;  # $parts[0] = ''
                                       # $parts[1] = '/'
                                       # $parts[2] = 'usr'
                                       # $parts[3] = '/'
                                       # $parts[4] = 'bin'
                                       # $parts[5] = '/'
                                       # $parts[6] = 'perl'

       Since the first character of $x matched the regexp,
       "split" prepended an empty initial element to the list.

       If you have read this far, congratulations! You now have
       all the basic tools needed to use regular expressions to
       solve a wide range of text processing problems.  If this
       is your first time through the tutorial, why not stop here
       and play around with regexps a while...  Part 2 concerns
       the more esoteric aspects of regular expressions and those
       concepts certainly aren't needed right at the start.





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Part 2: Power tools
       OK, you know the basics of regexps and you want to know
       more.  If matching regular expressions is analogous to a
       walk in the woods, then the tools discussed in Part 1 are
       analogous to topo maps and a compass, basic tools we use
       all the time.  Most of the tools in part 2 are analogous
       to flare guns and satellite phones.  They aren't used too
       often on a hike, but when we are stuck, they can be
       invaluable.

       What follows are the more advanced, less used, or
       sometimes esoteric capabilities of Perl regexps.  In Part
       2, we will assume you are comfortable with the basics and
       concentrate on the new features.

   More on characters, strings, and character classes
       There are a number of escape sequences and character
       classes that we haven't covered yet.

       There are several escape sequences that convert characters
       or strings between upper and lower case, and they are also
       available within patterns.  "\l" and "\u" convert the next
       character to lower or upper case, respectively:

           $x = "perl";
           $string =~ /\u$x/;  # matches 'Perl' in $string
           $x = "M(rs?|s)\\."; # note the double backslash
           $string =~ /\l$x/;  # matches 'mr.', 'mrs.', and 'ms.',

       A "\L" or "\U" indicates a lasting conversion of case,
       until terminated by "\E" or thrown over by another "\U" or
       "\L":

           $x = "This word is in lower case:\L SHOUT\E";
           $x =~ /shout/;       # matches
           $x = "I STILL KEYPUNCH CARDS FOR MY 360"
           $x =~ /\Ukeypunch/;  # matches punch card string

       If there is no "\E", case is converted until the end of
       the string. The regexps "\L\u$word" or "\u\L$word" convert
       the first character of $word to uppercase and the rest of
       the characters to lowercase.

       Control characters can be escaped with "\c", so that a
       control-Z character would be matched with "\cZ".  The
       escape sequence "\Q"..."\E" quotes, or protects most non-
       alphabetic characters.   For instance,

           $x = "\QThat !^*&%~& cat!";
           $x =~ /\Q!^*&%~&\E/;  # check for rough language

       It does not protect "$" or "@", so that variables can
       still be substituted.




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       With the advent of 5.6.0, Perl regexps can handle more
       than just the standard ASCII character set.  Perl now
       supports Unicode, a standard for representing the
       alphabets from virtually all of the world's written
       languages, and a host of symbols.  Perl's text strings are
       Unicode strings, so they can contain characters with a
       value (codepoint or character number) higher than 255

       What does this mean for regexps? Well, regexp users don't
       need to know much about Perl's internal representation of
       strings.  But they do need to know 1) how to represent
       Unicode characters in a regexp and 2) that a matching
       operation will treat the string to be searched as a
       sequence of characters, not bytes.  The answer to 1) is
       that Unicode characters greater than "chr(255)" are
       represented using the "\x{hex}" notation, because the \0
       octal and \x hex (without curly braces) don't go further
       than 255.

           /\x{263a}/;  # match a Unicode smiley face :)

       NOTE: In Perl 5.6.0 it used to be that one needed to say
       "use utf8" to use any Unicode features.  This is no more
       the case: for almost all Unicode processing, the explicit
       "utf8" pragma is not needed.  (The only case where it
       matters is if your Perl script is in Unicode and encoded
       in UTF-8, then an explicit "use utf8" is needed.)

       Figuring out the hexadecimal sequence of a Unicode
       character you want or deciphering someone else's
       hexadecimal Unicode regexp is about as much fun as
       programming in machine code.  So another way to specify
       Unicode characters is to use the named character> escape
       sequence "\N{name}".  "name" is a name for the Unicode
       character, as specified in the Unicode standard.  For
       instance, if we wanted to represent or match the
       astrological sign for the planet Mercury, we could use

           use charnames ":full"; # use named chars with Unicode full names
           $x = "abc\N{MERCURY}def";
           $x =~ /\N{MERCURY}/;   # matches

       One can also use short names or restrict names to a
       certain alphabet:

           use charnames ':full';
           print "\N{GREEK SMALL LETTER SIGMA} is called sigma.\n";

           use charnames ":short";
           print "\N{greek:Sigma} is an upper-case sigma.\n";

           use charnames qw(greek);
           print "\N{sigma} is Greek sigma\n";




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       A list of full names is found in the file NamesList.txt in
       the lib/perl5/X.X.X/unicore directory (where X.X.X is the
       perl version number as it is installed on your system).

       The answer to requirement 2), as of 5.6.0, is that a
       regexp uses Unicode characters. Internally, this is
       encoded to bytes using either UTF-8 or a native 8 bit
       encoding, depending on the history of the string, but
       conceptually it is a sequence of characters, not bytes.
       See perlunitut for a tutorial about that.

       Let us now discuss Unicode character classes.  Just as
       with Unicode characters, there are named Unicode character
       classes represented by the "\p{name}" escape sequence.
       Closely associated is the "\P{name}" character class,
       which is the negation of the "\p{name}" class.  For
       example, to match lower and uppercase characters,

           use charnames ":full"; # use named chars with Unicode full names
           $x = "BOB";
           $x =~ /^\p{IsUpper}/;   # matches, uppercase char class
           $x =~ /^\P{IsUpper}/;   # doesn't match, char class sans uppercase
           $x =~ /^\p{IsLower}/;   # doesn't match, lowercase char class
           $x =~ /^\P{IsLower}/;   # matches, char class sans lowercase

       Here is the association between some Perl named classes
       and the traditional Unicode classes:

           Perl class name  Unicode class name or regular expression

           IsAlpha          /^[LM]/
           IsAlnum          /^[LMN]/
           IsASCII          $code <= 127
           IsCntrl          /^C/
           IsBlank          $code =~ /^(0020|0009)$/ || /^Z[^lp]/
           IsDigit          Nd
           IsGraph          /^([LMNPS]|Co)/
           IsLower          Ll
           IsPrint          /^([LMNPS]|Co|Zs)/
           IsPunct          /^P/
           IsSpace          /^Z/ || ($code =~ /^(0009|000A|000B|000C|000D)$/
           IsSpacePerl      /^Z/ || ($code =~ /^(0009|000A|000C|000D|0085|2028|2029)$/
           IsUpper          /^L[ut]/
           IsWord           /^[LMN]/ || $code eq "005F"
           IsXDigit         $code =~ /^00(3[0-9]|[46][1-6])$/

       You can also use the official Unicode class names with the
       "\p" and "\P", like "\p{L}" for Unicode 'letters', or
       "\p{Lu}" for uppercase letters, or "\P{Nd}" for non-
       digits.  If a "name" is just one letter, the braces can be
       dropped.  For instance, "\pM" is the character class of
       Unicode 'marks', for example accent marks.  For the full
       list see perlunicode.




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       The Unicode has also been separated into various sets of
       characters which you can test with "\p{...}" (in) and
       "\P{...}" (not in).  To test whether a character is (or is
       not) an element of a script you would use the script name,
       for example "\p{Latin}", "\p{Greek}", or "\P{Katakana}".
       Other sets are the Unicode blocks, the names of which
       begin with "In". One such block is dedicated to
       mathematical operators, and its pattern formula is
       <C\p{InMathematicalOperators>}>.  For the full list see
       perlunicode.

       "\X" is an abbreviation for a character class that
       comprises the Unicode combining character sequences.  A
       combining character sequence is a base character followed
       by any number of diacritics, i.e., signs like accents used
       to indicate different sounds of a letter. Using the
       Unicode full names, e.g., "A + COMBINING RING" is a
       combining character sequence with base character "A" and
       combining character "COMBINING RING", which translates in
       Danish to A with the circle atop it, as in the word
       Angstrom.  "\X" is equivalent to "\PM\pM*}", i.e., a non-
       mark followed by one or more marks.

       For the full and latest information about Unicode see the
       latest Unicode standard, or the Unicode Consortium's
       website http://www.unicode.org/

       As if all those classes weren't enough, Perl also defines
       POSIX style character classes.  These have the form
       "[:name:]", with "name" the name of the POSIX class.  The
       POSIX classes are "alpha", "alnum", "ascii", "cntrl",
       "digit", "graph", "lower", "print", "punct", "space",
       "upper", and "xdigit", and two extensions, "word" (a Perl
       extension to match "\w"), and "blank" (a GNU extension).
       If "utf8" is being used, then these classes are defined
       the same as their corresponding Perl Unicode classes:
       "[:upper:]" is the same as "\p{IsUpper}", etc.  The POSIX
       character classes, however, don't require using "utf8".
       The "[:digit:]", "[:word:]", and "[:space:]" correspond to
       the familiar "\d", "\w", and "\s" character classes.  To
       negate a POSIX class, put a "^" in front of the name, so
       that, e.g., "[:^digit:]" corresponds to "\D" and under
       "utf8", "\P{IsDigit}".  The Unicode and POSIX character
       classes can be used just like "\d", with the exception
       that POSIX character classes can only be used inside of a
       character class:

           /\s+[abc[:digit:]xyz]\s*/;  # match a,b,c,x,y,z, or a digit
           /^=item\s[[:digit:]]/;      # match '=item',
                                       # followed by a space and a digit
           use charnames ":full";
           /\s+[abc\p{IsDigit}xyz]\s+/;  # match a,b,c,x,y,z, or a digit
           /^=item\s\p{IsDigit}/;        # match '=item',
                                         # followed by a space and a digit



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       Whew! That is all the rest of the characters and character
       classes.

   Compiling and saving regular expressions
       In Part 1 we discussed the "//o" modifier, which compiles
       a regexp just once.  This suggests that a compiled regexp
       is some data structure that can be stored once and used
       again and again.  The regexp quote "qr//" does exactly
       that: "qr/string/" compiles the "string" as a regexp and
       transforms the result into a form that can be assigned to
       a variable:

           $reg = qr/foo+bar?/;  # reg contains a compiled regexp

       Then $reg can be used as a regexp:

           $x = "fooooba";
           $x =~ $reg;     # matches, just like /foo+bar?/
           $x =~ /$reg/;   # same thing, alternate form

       $reg can also be interpolated into a larger regexp:

           $x =~ /(abc)?$reg/;  # still matches

       As with the matching operator, the regexp quote can use
       different delimiters, e.g., "qr!!", "qr{}" or "qr~~".
       Apostrophes as delimiters ("qr''") inhibit any
       interpolation.

       Pre-compiled regexps are useful for creating dynamic
       matches that don't need to be recompiled each time they
       are encountered.  Using pre-compiled regexps, we write a
       "grep_step" program which greps for a sequence of
       patterns, advancing to the next pattern as soon as one has
       been satisfied.

           % cat > grep_step
           #!/usr/bin/perl
           # grep_step - match <number> regexps, one after the other
           # usage: multi_grep <number> regexp1 regexp2 ... file1 file2 ...

           $number = shift;
           $regexp[$_] = shift foreach (0..$number-1);
           @compiled = map qr/$_/, @regexp;
           while ($line = <>) {
               if ($line =~ /$compiled[0]/) {
                   print $line;
                   shift @compiled;
                   last unless @compiled;
               }
           }
           ^D

           % grep_step 3 shift print last grep_step



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           $number = shift;
                   print $line;
                   last unless @compiled;

       Storing pre-compiled regexps in an array @compiled allows
       us to simply loop through the regexps without any
       recompilation, thus gaining flexibility without
       sacrificing speed.

   Composing regular expressions at runtime
       Backtracking is more efficient than repeated tries with
       different regular expressions.  If there are several
       regular expressions and a match with any of them is
       acceptable, then it is possible to combine them into a set
       of alternatives.  If the individual expressions are input
       data, this can be done by programming a join operation.
       We'll exploit this idea in an improved version of the
       "simple_grep" program: a program that matches multiple
       patterns:

           % cat > multi_grep
           #!/usr/bin/perl
           # multi_grep - match any of <number> regexps
           # usage: multi_grep <number> regexp1 regexp2 ... file1 file2 ...

           $number = shift;
           $regexp[$_] = shift foreach (0..$number-1);
           $pattern = join '|', @regexp;

           while ($line = <>) {
               print $line if $line =~ /$pattern/o;
           }
           ^D

           % multi_grep 2 shift for multi_grep
           $number = shift;
           $regexp[$_] = shift foreach (0..$number-1);

       Sometimes it is advantageous to construct a pattern from
       the input that is to be analyzed and use the permissible
       values on the left hand side of the matching operations.
       As an example for this somewhat paradoxical situation,
       let's assume that our input contains a command verb which
       should match one out of a set of available command verbs,
       with the additional twist that commands may be abbreviated
       as long as the given string is unique. The program below
       demonstrates the basic algorithm.










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           % cat > keymatch
           #!/usr/bin/perl
           $kwds = 'copy compare list print';
           while( $command = <> ){
               $command =~ s/^\s+|\s+$//g;  # trim leading and trailing spaces
               if( ( @matches = $kwds =~ /\b$command\w*/g ) == 1 ){
                   print "command: '@matches'\n";
               } elsif( @matches == 0 ){
                   print "no such command: '$command'\n";
               } else {
                   print "not unique: '$command' (could be one of: @matches)\n";
               }
           }
           ^D

           % keymatch
           li
           command: 'list'
           co
           not unique: 'co' (could be one of: copy compare)
           printer
           no such command: 'printer'

       Rather than trying to match the input against the
       keywords, we match the combined set of keywords against
       the input.  The pattern matching operation
       "$kwds =~ /\b($command\w*)/g" does several things at the
       same time. It makes sure that the given command begins
       where a keyword begins ("\b"). It tolerates abbreviations
       due to the added "\w*". It tells us the number of matches
       ("scalar @matches") and all the keywords that were
       actually matched.  You could hardly ask for more.

   Embedding comments and modifiers in a regular expression
       Starting with this section, we will be discussing Perl's
       set of extended patterns.  These are extensions to the
       traditional regular expression syntax that provide
       powerful new tools for pattern matching.  We have already
       seen extensions in the form of the minimal matching
       constructs "??", "*?", "+?", "{n,m}?", and "{n,}?".  The
       rest of the extensions below have the form "(?char...)",
       where the "char" is a character that determines the type
       of extension.

       The first extension is an embedded comment "(?#text)".
       This embeds a comment into the regular expression without
       affecting its meaning.  The comment should not have any
       closing parentheses in the text.  An example is

           /(?# Match an integer:)[+-]?\d+/;

       This style of commenting has been largely superseded by
       the raw, freeform commenting that is allowed with the
       "//x" modifier.



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       The modifiers "//i", "//m", "//s" and "//x" (or any
       combination thereof) can also be embedded in a regexp
       using "(?i)", "(?m)", "(?s)", and "(?x)".  For instance,

           /(?i)yes/;  # match 'yes' case insensitively
           /yes/i;     # same thing
           /(?x)(          # freeform version of an integer regexp
                    [+-]?  # match an optional sign
                    \d+    # match a sequence of digits
                )
           /x;

       Embedded modifiers can have two important advantages over
       the usual modifiers.  Embedded modifiers allow a custom
       set of modifiers to each regexp pattern.  This is great
       for matching an array of regexps that must have different
       modifiers:

           $pattern[0] = '(?i)doctor';
           $pattern[1] = 'Johnson';
           ...
           while (<>) {
               foreach $patt (@pattern) {
                   print if /$patt/;
               }
           }

       The second advantage is that embedded modifiers (except
       "//p", which modifies the entire regexp) only affect the
       regexp inside the group the embedded modifier is contained
       in.  So grouping can be used to localize the modifier's
       effects:

           /Answer: ((?i)yes)/;  # matches 'Answer: yes', 'Answer: YES', etc.

       Embedded modifiers can also turn off any modifiers already
       present by using, e.g., "(?-i)".  Modifiers can also be
       combined into a single expression, e.g., "(?s-i)" turns on
       single line mode and turns off case insensitivity.

       Embedded modifiers may also be added to a non-capturing
       grouping.  "(?i-m:regexp)" is a non-capturing grouping
       that matches "regexp" case insensitively and turns off
       multi-line mode.

   Looking ahead and looking behind
       This section concerns the lookahead and lookbehind
       assertions.  First, a little background.

       In Perl regular expressions, most regexp elements 'eat up'
       a certain amount of string when they match.  For instance,
       the regexp element "[abc}]" eats up one character of the
       string when it matches, in the sense that Perl moves to
       the next character position in the string after the match.



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       There are some elements, however, that don't eat up
       characters (advance the character position) if they match.
       The examples we have seen so far are the anchors.  The
       anchor "^" matches the beginning of the line, but doesn't
       eat any characters.  Similarly, the word boundary anchor
       "\b" matches wherever a character matching "\w" is next to
       a character that doesn't, but it doesn't eat up any
       characters itself.  Anchors are examples of zero-width
       assertions.  Zero-width, because they consume no
       characters, and assertions, because they test some
       property of the string.  In the context of our walk in the
       woods analogy to regexp matching, most regexp elements
       move us along a trail, but anchors have us stop a moment
       and check our surroundings.  If the local environment
       checks out, we can proceed forward.  But if the local
       environment doesn't satisfy us, we must backtrack.

       Checking the environment entails either looking ahead on
       the trail, looking behind, or both.  "^" looks behind, to
       see that there are no characters before.  "$" looks ahead,
       to see that there are no characters after.  "\b" looks
       both ahead and behind, to see if the characters on either
       side differ in their "word-ness".

       The lookahead and lookbehind assertions are
       generalizations of the anchor concept.  Lookahead and
       lookbehind are zero-width assertions that let us specify
       which characters we want to test for.  The lookahead
       assertion is denoted by "(?=regexp)" and the lookbehind
       assertion is denoted by "(?<=fixed-regexp)".  Some
       examples are

           $x = "I catch the housecat 'Tom-cat' with catnip";
           $x =~ /cat(?=\s)/;   # matches 'cat' in 'housecat'
           @catwords = ($x =~ /(?<=\s)cat\w+/g);  # matches,
                                                  # $catwords[0] = 'catch'
                                                  # $catwords[1] = 'catnip'
           $x =~ /\bcat\b/;  # matches 'cat' in 'Tom-cat'
           $x =~ /(?<=\s)cat(?=\s)/; # doesn't match; no isolated 'cat' in
                                     # middle of $x

       Note that the parentheses in "(?=regexp)" and
       "(?<=regexp)" are non-capturing, since these are zero-
       width assertions.  Thus in the second regexp, the
       substrings captured are those of the whole regexp itself.
       Lookahead "(?=regexp)" can match arbitrary regexps, but
       lookbehind "(?<=fixed-regexp)" only works for regexps of
       fixed width, i.e., a fixed number of characters long.
       Thus "(?<=(ab|bc))" is fine, but "(?<=(ab)*)" is not.  The
       negated versions of the lookahead and lookbehind
       assertions are denoted by "(?!regexp)" and
       "(?<!fixed-regexp)" respectively.  They evaluate true if
       the regexps do not match:




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           $x = "foobar";
           $x =~ /foo(?!bar)/;  # doesn't match, 'bar' follows 'foo'
           $x =~ /foo(?!baz)/;  # matches, 'baz' doesn't follow 'foo'
           $x =~ /(?<!\s)foo/;  # matches, there is no \s before 'foo'

       The "\C" is unsupported in lookbehind, because the already
       treacherous definition of "\C" would become even more so
       when going backwards.

       Here is an example where a string containing blank-
       separated words, numbers and single dashes is to be split
       into its components.  Using "/\s+/" alone won't work,
       because spaces are not required between dashes, or a word
       or a dash. Additional places for a split are established
       by looking ahead and behind:

           $str = "one two - --6-8";
           @toks = split / \s+              # a run of spaces
                         | (?<=\S) (?=-)    # any non-space followed by '-'
                         | (?<=-)  (?=\S)   # a '-' followed by any non-space
                         /x, $str;          # @toks = qw(one two - - - 6 - 8)

   Using independent subexpressions to prevent backtracking
       Independent subexpressions are regular expressions, in the
       context of a larger regular expression, that function
       independently of the larger regular expression.  That is,
       they consume as much or as little of the string as they
       wish without regard for the ability of the larger regexp
       to match.  Independent subexpressions are represented by
       "(?>regexp)".  We can illustrate their behavior by first
       considering an ordinary regexp:

           $x = "ab";
           $x =~ /a*ab/;  # matches

       This obviously matches, but in the process of matching,
       the subexpression "a*" first grabbed the "a".  Doing so,
       however, wouldn't allow the whole regexp to match, so
       after backtracking, "a*" eventually gave back the "a" and
       matched the empty string.  Here, what "a*" matched was
       dependent on what the rest of the regexp matched.

       Contrast that with an independent subexpression:

           $x =~ /(?>a*)ab/;  # doesn't match!

       The independent subexpression "(?>a*)" doesn't care about
       the rest of the regexp, so it sees an "a" and grabs it.
       Then the rest of the regexp "ab" cannot match.  Because
       "(?>a*)" is independent, there is no backtracking and the
       independent subexpression does not give up its "a".  Thus
       the match of the regexp as a whole fails.  A similar
       behavior occurs with completely independent regexps:




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           $x = "ab";
           $x =~ /a*/g;   # matches, eats an 'a'
           $x =~ /\Gab/g; # doesn't match, no 'a' available

       Here "//g" and "\G" create a 'tag team' handoff of the
       string from one regexp to the other.  Regexps with an
       independent subexpression are much like this, with a
       handoff of the string to the independent subexpression,
       and a handoff of the string back to the enclosing regexp.

       The ability of an independent subexpression to prevent
       backtracking can be quite useful.  Suppose we want to
       match a non-empty string enclosed in parentheses up to two
       levels deep.  Then the following regexp matches:

           $x = "abc(de(fg)h";  # unbalanced parentheses
           $x =~ /\( ( [^()]+ | \([^()]*\) )+ \)/x;

       The regexp matches an open parenthesis, one or more copies
       of an alternation, and a close parenthesis.  The
       alternation is two-way, with the first alternative
       "[^()]+" matching a substring with no parentheses and the
       second alternative "\([^()]*\)"  matching a substring
       delimited by parentheses.  The problem with this regexp is
       that it is pathological: it has nested indeterminate
       quantifiers of the form "(a+|b)+".  We discussed in Part 1
       how nested quantifiers like this could take an
       exponentially long time to execute if there was no match
       possible.  To prevent the exponential blowup, we need to
       prevent useless backtracking at some point.  This can be
       done by enclosing the inner quantifier as an independent
       subexpression:

           $x =~ /\( ( (?>[^()]+) | \([^()]*\) )+ \)/x;

       Here, "(?>[^()]+)" breaks the degeneracy of string
       partitioning by gobbling up as much of the string as
       possible and keeping it.   Then match failures fail much
       more quickly.

   Conditional expressions
       A conditional expression is a form of if-then-else
       statement that allows one to choose which patterns are to
       be matched, based on some condition.  There are two types
       of conditional expression: "(?(condition)yes-regexp)" and
       "(?(condition)yes-regexp|no-regexp)".
       "(?(condition)yes-regexp)" is like an 'if () {}' statement
       in Perl.  If the "condition" is true, the "yes-regexp"
       will be matched.  If the "condition" is false, the
       "yes-regexp" will be skipped and Perl will move onto the
       next regexp element.  The second form is like an
       'if () {} else {}' statement in Perl.  If the "condition"
       is true, the "yes-regexp" will be matched, otherwise the
       "no-regexp" will be matched.



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       The "condition" can have several forms.  The first form is
       simply an integer in parentheses "(integer)".  It is true
       if the corresponding backreference "\integer" matched
       earlier in the regexp.  The same thing can be done with a
       name associated with a capture buffer, written as
       "(<name>)" or "('name')".  The second form is a bare zero
       width assertion "(?...)", either a lookahead, a
       lookbehind, or a code assertion (discussed in the next
       section).  The third set of forms provides tests that
       return true if the expression is executed within a
       recursion ("(R)") or is being called from some capturing
       group, referenced either by number ("(R1)", "(R2)",...) or
       by name ("(R&name)").

       The integer or name form of the "condition" allows us to
       choose, with more flexibility, what to match based on what
       matched earlier in the regexp. This searches for words of
       the form "$x$x" or "$x$y$y$x":

           % simple_grep '^(\w+)(\w+)?(?(2)\2\1|\1)$' /usr/dict/words
           beriberi
           coco
           couscous
           deed
           ...
           toot
           toto
           tutu

       The lookbehind "condition" allows, along with
       backreferences, an earlier part of the match to influence
       a later part of the match.  For instance,

           /[ATGC]+(?(?<=AA)G|C)$/;

       matches a DNA sequence such that it either ends in "AAG",
       or some other base pair combination and "C".  Note that
       the form is "(?(?<=AA)G|C)" and not "(?((?<=AA))G|C)"; for
       the lookahead, lookbehind or code assertions, the
       parentheses around the conditional are not needed.

   Defining named patterns
       Some regular expressions use identical subpatterns in
       several places.  Starting with Perl 5.10, it is possible
       to define named subpatterns in a section of the pattern so
       that they can be called up by name anywhere in the
       pattern.  This syntactic pattern for this definition group
       is "(?(DEFINE)(?<name>pattern)...)".  An insertion of a
       named pattern is written as "(?&name)".

       The example below illustrates this feature using the
       pattern for floating point numbers that was presented
       earlier on.  The three subpatterns that are used more than
       once are the optional sign, the digit sequence for an



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       integer and the decimal fraction.  The DEFINE group at the
       end of the pattern contains their definition.  Notice that
       the decimal fraction pattern is the first place where we
       can reuse the integer pattern.

          /^ (?&osg)\ * ( (?&int)(?&dec)? | (?&dec) )
             (?: [eE](?&osg)(?&int) )?
           $
           (?(DEFINE)
             (?<osg>[-+]?)         # optional sign
             (?<int>\d++)          # integer
             (?<dec>\.(?&int))     # decimal fraction
           )/x

   Recursive patterns
       This feature (introduced in Perl 5.10) significantly
       extends the power of Perl's pattern matching.  By
       referring to some other capture group anywhere in the
       pattern with the construct "(?group-ref)", the pattern
       within the referenced group is used as an independent
       subpattern in place of the group reference itself.
       Because the group reference may be contained within the
       group it refers to, it is now possible to apply pattern
       matching to tasks that hitherto required a recursive
       parser.

       To illustrate this feature, we'll design a pattern that
       matches if a string contains a palindrome. (This is a word
       or a sentence that, while ignoring spaces,
       interpunctuation and case, reads the same backwards as
       forwards. We begin by observing that the empty string or a
       string containing just one word character is a palindrome.
       Otherwise it must have a word character up front and the
       same at its end, with another palindrome in between.

           /(?: (\w) (?...Here be a palindrome...) \g{-1} | \w? )/x

       Adding "\W*" at either end to eliminate what is to be
       ignored, we already have the full pattern:

           my $pp = qr/^(\W* (?: (\w) (?1) \g{-1} | \w? ) \W*)$/ix;
           for $s ( "saippuakauppias", "A man, a plan, a canal: Panama!" ){
               print "'$s' is a palindrome\n" if $s =~ /$pp/;
           }

       In "(?...)" both absolute and relative backreferences may
       be used.  The entire pattern can be reinserted with "(?R)"
       or "(?0)".  If you prefer to name your buffers, you can
       use "(?&name)" to recurse into that buffer.

   A bit of magic: executing Perl code in a regular expression
       Normally, regexps are a part of Perl expressions.  Code
       evaluation expressions turn that around by allowing
       arbitrary Perl code to be a part of a regexp.  A code



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       evaluation expression is denoted "(?{code})", with code a
       string of Perl statements.

       Be warned that this feature is considered experimental,
       and may be changed without notice.

       Code expressions are zero-width assertions, and the value
       they return depends on their environment.  There are two
       possibilities: either the code expression is used as a
       conditional in a conditional expression
       "(?(condition)...)", or it is not.  If the code expression
       is a conditional, the code is evaluated and the result
       (i.e., the result of the last statement) is used to
       determine truth or falsehood.  If the code expression is
       not used as a conditional, the assertion always evaluates
       true and the result is put into the special variable $^R.
       The variable $^R can then be used in code expressions
       later in the regexp.  Here are some silly examples:

           $x = "abcdef";
           $x =~ /abc(?{print "Hi Mom!";})def/; # matches,
                                                # prints 'Hi Mom!'
           $x =~ /aaa(?{print "Hi Mom!";})def/; # doesn't match,
                                                # no 'Hi Mom!'

       Pay careful attention to the next example:

           $x =~ /abc(?{print "Hi Mom!";})ddd/; # doesn't match,
                                                # no 'Hi Mom!'
                                                # but why not?

       At first glance, you'd think that it shouldn't print,
       because obviously the "ddd" isn't going to match the
       target string. But look at this example:

           $x =~ /abc(?{print "Hi Mom!";})[d]dd/; # doesn't match,
                                                  # but _does_ print

       Hmm. What happened here? If you've been following along,
       you know that the above pattern should be effectively the
       same as the last one -- enclosing the d in a character
       class isn't going to change what it matches. So why does
       the first not print while the second one does?

       The answer lies in the optimizations the regex engine
       makes. In the first case, all the engine sees are plain
       old characters (aside from the "?{}" construct). It's
       smart enough to realize that the string 'ddd' doesn't
       occur in our target string before actually running the
       pattern through. But in the second case, we've tricked it
       into thinking that our pattern is more complicated than it
       is. It takes a look, sees our character class, and decides
       that it will have to actually run the pattern to determine
       whether or not it matches, and in the process of running



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       it hits the print statement before it discovers that we
       don't have a match.

       To take a closer look at how the engine does
       optimizations, see the section "Pragmas and debugging"
       below.

       More fun with "?{}":

           $x =~ /(?{print "Hi Mom!";})/;       # matches,
                                                # prints 'Hi Mom!'
           $x =~ /(?{$c = 1;})(?{print "$c";})/;  # matches,
                                                  # prints '1'
           $x =~ /(?{$c = 1;})(?{print "$^R";})/; # matches,
                                                  # prints '1'

       The bit of magic mentioned in the section title occurs
       when the regexp backtracks in the process of searching for
       a match.  If the regexp backtracks over a code expression
       and if the variables used within are localized using
       "local", the changes in the variables produced by the code
       expression are undone! Thus, if we wanted to count how
       many times a character got matched inside a group, we
       could use, e.g.,

           $x = "aaaa";
           $count = 0;  # initialize 'a' count
           $c = "bob";  # test if $c gets clobbered
           $x =~ /(?{local $c = 0;})         # initialize count
                  ( a                        # match 'a'
                    (?{local $c = $c + 1;})  # increment count
                  )*                         # do this any number of times,
                  aa                         # but match 'aa' at the end
                  (?{$count = $c;})          # copy local $c var into $count
                 /x;
           print "'a' count is $count, \$c variable is '$c'\n";

       This prints

           'a' count is 2, $c variable is 'bob'

       If we replace the " (?{local $c = $c + 1;})" with
       " (?{$c = $c + 1;})", the variable changes are not undone
       during backtracking, and we get

           'a' count is 4, $c variable is 'bob'

       Note that only localized variable changes are undone.
       Other side effects of code expression execution are
       permanent.  Thus

           $x = "aaaa";
           $x =~ /(a(?{print "Yow\n";}))*aa/;




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       produces

          Yow
          Yow
          Yow
          Yow

       The result $^R is automatically localized, so that it will
       behave properly in the presence of backtracking.

       This example uses a code expression in a conditional to
       match a definite article, either 'the' in English or
       'der|die|das' in German:

           $lang = 'DE';  # use German
           ...
           $text = "das";
           print "matched\n"
               if $text =~ /(?(?{
                                 $lang eq 'EN'; # is the language English?
                                })
                              the |             # if so, then match 'the'
                              (der|die|das)     # else, match 'der|die|das'
                            )
                           /xi;

       Note that the syntax here is
       "(?(?{...})yes-regexp|no-regexp)", not
       "(?((?{...}))yes-regexp|no-regexp)".  In other words, in
       the case of a code expression, we don't need the extra
       parentheses around the conditional.

       If you try to use code expressions with interpolating
       variables, Perl may surprise you:

           $bar = 5;
           $pat = '(?{ 1 })';
           /foo(?{ $bar })bar/; # compiles ok, $bar not interpolated
           /foo(?{ 1 })$bar/;   # compile error!
           /foo${pat}bar/;      # compile error!

           $pat = qr/(?{ $foo = 1 })/;  # precompile code regexp
           /foo${pat}bar/;      # compiles ok

       If a regexp has (1) code expressions and interpolating
       variables, or (2) a variable that interpolates a code
       expression, Perl treats the regexp as an error. If the
       code expression is precompiled into a variable, however,
       interpolating is ok. The question is, why is this an
       error?

       The reason is that variable interpolation and code
       expressions together pose a security risk.  The
       combination is dangerous because many programmers who



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       write search engines often take user input and plug it
       directly into a regexp:

           $regexp = <>;       # read user-supplied regexp
           $chomp $regexp;     # get rid of possible newline
           $text =~ /$regexp/; # search $text for the $regexp

       If the $regexp variable contains a code expression, the
       user could then execute arbitrary Perl code.  For
       instance, some joker could search for
       "system('rm -rf *');" to erase your files.  In this sense,
       the combination of interpolation and code expressions
       taints your regexp.  So by default, using both
       interpolation and code expressions in the same regexp is
       not allowed.  If you're not concerned about malicious
       users, it is possible to bypass this security check by
       invoking "use re 'eval'":

           use re 'eval';       # throw caution out the door
           $bar = 5;
           $pat = '(?{ 1 })';
           /foo(?{ 1 })$bar/;   # compiles ok
           /foo${pat}bar/;      # compiles ok

       Another form of code expression is the pattern code
       expression.  The pattern code expression is like a regular
       code expression, except that the result of the code
       evaluation is treated as a regular expression and matched
       immediately.  A simple example is

           $length = 5;
           $char = 'a';
           $x = 'aaaaabb';
           $x =~ /(??{$char x $length})/x; # matches, there are 5 of 'a'

       This final example contains both ordinary and pattern code
       expressions.  It detects whether a binary string
       1101010010001... has a Fibonacci spacing 0,1,1,2,3,5,...
       of the 1's:

           $x = "1101010010001000001";
           $z0 = ''; $z1 = '0';   # initial conditions
           print "It is a Fibonacci sequence\n"
               if $x =~ /^1         # match an initial '1'
                           (?:
                              ((??{ $z0 })) # match some '0'
                              1             # and then a '1'
                              (?{ $z0 = $z1; $z1 .= $^N; })
                           )+   # repeat as needed
                         $      # that is all there is
                        /x;
           printf "Largest sequence matched was %d\n", length($z1)-length($z0);

       Remember that $^N is set to whatever was matched by the



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       last completed capture group. This prints

           It is a Fibonacci sequence
           Largest sequence matched was 5

       Ha! Try that with your garden variety regexp package...

       Note that the variables $z0 and $z1 are not substituted
       when the regexp is compiled, as happens for ordinary
       variables outside a code expression.  Rather, the code
       expressions are evaluated when Perl encounters them during
       the search for a match.

       The regexp without the "//x" modifier is

           /^1(?:((??{ $z0 }))1(?{ $z0 = $z1; $z1 .= $^N; }))+$/

       which shows that spaces are still possible in the code
       parts. Nevertheless, when working with code and
       conditional expressions, the extended form of regexps is
       almost necessary in creating and debugging regexps.

   Backtracking control verbs
       Perl 5.10 introduced a number of control verbs intended to
       provide detailed control over the backtracking process, by
       directly influencing the regexp engine and by providing
       monitoring techniques.  As all the features in this group
       are experimental and subject to change or removal in a
       future version of Perl, the interested reader is referred
       to "Special Backtracking Control Verbs" in perlre for a
       detailed description.

       Below is just one example, illustrating the control verb
       "(*FAIL)", which may be abbreviated as "(*F)". If this is
       inserted in a regexp it will cause to fail, just like at
       some mismatch between the pattern and the string.
       Processing of the regexp continues like after any "normal"
       failure, so that, for instance, the next position in the
       string or another alternative will be tried. As failing to
       match doesn't preserve capture buffers or produce results,
       it may be necessary to use this in combination with
       embedded code.

          %count = ();
          "supercalifragilisticexpialidoceous" =~
              /([aeiou])(?{ $count{$1}++; })(*FAIL)/oi;
          printf "%3d '%s'\n", $count{$_}, $_ for (sort keys %count);

       The pattern begins with a class matching a subset of
       letters.  Whenever this matches, a statement like
       "$count{'a'}++;" is executed, incrementing the letter's
       counter. Then "(*FAIL)" does what it says, and the regexp
       engine proceeds according to the book: as long as the end
       of the string  hasn't been reached, the position is



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       advanced before looking for another vowel. Thus, match or
       no match makes no difference, and the regexp engine
       proceeds until the the entire string has been inspected.
       (It's remarkable that an alternative solution using
       something like

          $count{lc($_)}++ for split('', "supercalifragilisticexpialidoceous");
          printf "%3d '%s'\n", $count2{$_}, $_ for ( qw{ a e i o u } );

       is considerably slower.)

   Pragmas and debugging
       Speaking of debugging, there are several pragmas available
       to control and debug regexps in Perl.  We have already
       encountered one pragma in the previous section,
       "use re 'eval';", that allows variable interpolation and
       code expressions to coexist in a regexp.  The other
       pragmas are

           use re 'taint';
           $tainted = <>;
           @parts = ($tainted =~ /(\w+)\s+(\w+)/; # @parts is now tainted

       The "taint" pragma causes any substrings from a match with
       a tainted variable to be tainted as well.  This is not
       normally the case, as regexps are often used to extract
       the safe bits from a tainted variable.  Use "taint" when
       you are not extracting safe bits, but are performing some
       other processing.  Both "taint" and "eval" pragmas are
       lexically scoped, which means they are in effect only
       until the end of the block enclosing the pragmas.

           use re 'debug';
           /^(.*)$/s;       # output debugging info

           use re 'debugcolor';
           /^(.*)$/s;       # output debugging info in living color

       The global "debug" and "debugcolor" pragmas allow one to
       get detailed debugging info about regexp compilation and
       execution.  "debugcolor" is the same as debug, except the
       debugging information is displayed in color on terminals
       that can display termcap color sequences.  Here is example
       output:













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           % perl -e 'use re "debug"; "abc" =~ /a*b+c/;'
           Compiling REx `a*b+c'
           size 9 first at 1
              1: STAR(4)
              2:   EXACT <a>(0)
              4: PLUS(7)
              5:   EXACT <b>(0)
              7: EXACT <c>(9)
              9: END(0)
           floating `bc' at 0..2147483647 (checking floating) minlen 2
           Guessing start of match, REx `a*b+c' against `abc'...
           Found floating substr `bc' at offset 1...
           Guessed: match at offset 0
           Matching REx `a*b+c' against `abc'
             Setting an EVAL scope, savestack=3
              0 <> <abc>             |  1:  STAR
                                      EXACT <a> can match 1 times out of 32767...
             Setting an EVAL scope, savestack=3
              1 <a> <bc>             |  4:    PLUS
                                      EXACT <b> can match 1 times out of 32767...
             Setting an EVAL scope, savestack=3
              2 <ab> <c>             |  7:      EXACT <c>
              3 <abc> <>             |  9:      END
           Match successful!
           Freeing REx: `a*b+c'

       If you have gotten this far into the tutorial, you can
       probably guess what the different parts of the debugging
       output tell you.  The first part

           Compiling REx `a*b+c'
           size 9 first at 1
              1: STAR(4)
              2:   EXACT <a>(0)
              4: PLUS(7)
              5:   EXACT <b>(0)
              7: EXACT <c>(9)
              9: END(0)

       describes the compilation stage.  STAR(4) means that there
       is a starred object, in this case 'a', and if it matches,
       goto line 4, i.e., PLUS(7).  The middle lines describe
       some heuristics and optimizations performed before a
       match:

           floating `bc' at 0..2147483647 (checking floating) minlen 2
           Guessing start of match, REx `a*b+c' against `abc'...
           Found floating substr `bc' at offset 1...
           Guessed: match at offset 0

       Then the match is executed and the remaining lines
       describe the process:





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           Matching REx `a*b+c' against `abc'
             Setting an EVAL scope, savestack=3
              0 <> <abc>             |  1:  STAR
                                      EXACT <a> can match 1 times out of 32767...
             Setting an EVAL scope, savestack=3
              1 <a> <bc>             |  4:    PLUS
                                      EXACT <b> can match 1 times out of 32767...
             Setting an EVAL scope, savestack=3
              2 <ab> <c>             |  7:      EXACT <c>
              3 <abc> <>             |  9:      END
           Match successful!
           Freeing REx: `a*b+c'

       Each step is of the form "n <x> <y>", with "<x>" the part
       of the string matched and "<y>" the part not yet matched.
       The "|  1:  STAR" says that Perl is at line number 1 n the
       compilation list above.  See "Debugging regular
       expressions" in perldebguts for much more detail.

       An alternative method of debugging regexps is to embed
       "print" statements within the regexp.  This provides a
       blow-by-blow account of the backtracking in an
       alternation:

           "that this" =~ m@(?{print "Start at position ", pos, "\n";})
                            t(?{print "t1\n";})
                            h(?{print "h1\n";})
                            i(?{print "i1\n";})
                            s(?{print "s1\n";})
                                |
                            t(?{print "t2\n";})
                            h(?{print "h2\n";})
                            a(?{print "a2\n";})
                            t(?{print "t2\n";})
                            (?{print "Done at position ", pos, "\n";})
                           @x;

       prints

           Start at position 0
           t1
           h1
           t2
           h2
           a2
           t2
           Done at position 4

BUGS
       Code expressions, conditional expressions, and independent
       expressions are experimental.  Don't use them in
       production code.  Yet.





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SEE ALSO
       This is just a tutorial.  For the full story on Perl
       regular expressions, see the perlre regular expressions
       reference page.

       For more information on the matching "m//" and
       substitution "s///" operators, see "Regexp Quote-Like
       Operators" in perlop.  For information on the "split"
       operation, see "split" in perlfunc.

       For an excellent all-around resource on the care and
       feeding of regular expressions, see the book Mastering
       Regular Expressions by Jeffrey Friedl (published by
       O'Reilly, ISBN 1556592-257-3).

AUTHOR AND COPYRIGHT
       Copyright (c) 2000 Mark Kvale All rights reserved.

       This document may be distributed under the same terms as
       Perl itself.

   Acknowledgments
       The inspiration for the stop codon DNA example came from
       the ZIP code example in chapter 7 of Mastering Regular
       Expressions.

       The author would like to thank Jeff Pinyan, Andrew
       Johnson, Peter Haworth, Ronald J Kimball, and Joe Smith
       for all their helpful comments.




























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