


PERLOP(1)        Perl Programmers Reference Guide       PERLOP(1)


NAME
       perlop - Perl operators and precedence

DESCRIPTION
   Operator Precedence and Associativity
       Operator precedence and associativity work in Perl more or
       less like they do in mathematics.

       Operator precedence means some operators are evaluated
       before others.  For example, in "2 + 4 * 5", the
       multiplication has higher precedence so "4 * 5" is
       evaluated first yielding "2 + 20 == 22" and not "6 * 5 ==
       30".

       Operator associativity defines what happens if a sequence
       of the same operators is used one after another: whether
       the evaluator will evaluate the left operations first or
       the right.  For example, in "8 - 4 - 2", subtraction is
       left associative so Perl evaluates the expression left to
       right.  "8 - 4" is evaluated first making the expression
       "4 - 2 == 2" and not "8 - 2 == 6".

       Perl operators have the following associativity and
       precedence, listed from highest precedence to lowest.
       Operators borrowed from C keep the same precedence
       relationship with each other, even where C's precedence is
       slightly screwy.  (This makes learning Perl easier for C
       folks.)  With very few exceptions, these all operate on
       scalar values only, not array values.

           left        terms and list operators (leftward)
           left        ->
           nonassoc    ++ --
           right       **
           right       ! ~ \ and unary + and -
           left        =~ !~
           left        * / % x
           left        + - .
           left        << >>
           nonassoc    named unary operators
           nonassoc    < > <= >= lt gt le ge
           nonassoc    == != <=> eq ne cmp ~~
           left        &
           left        | ^
           left        &&
           left        || //
           nonassoc    ..  ...
           right       ?:
           right       = += -= *= etc.
           left        , =>
           nonassoc    list operators (rightward)
           right       not
           left        and
           left        or xor



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       In the following sections, these operators are covered in
       precedence order.

       Many operators can be overloaded for objects.  See
       overload.

   Terms and List Operators (Leftward)
       A TERM has the highest precedence in Perl.  They include
       variables, quote and quote-like operators, any expression
       in parentheses, and any function whose arguments are
       parenthesized.  Actually, there aren't really functions in
       this sense, just list operators and unary operators
       behaving as functions because you put parentheses around
       the arguments.  These are all documented in perlfunc.

       If any list operator (print(), etc.) or any unary operator
       (chdir(), etc.)  is followed by a left parenthesis as the
       next token, the operator and arguments within parentheses
       are taken to be of highest precedence, just like a normal
       function call.

       In the absence of parentheses, the precedence of list
       operators such as "print", "sort", or "chmod" is either
       very high or very low depending on whether you are looking
       at the left side or the right side of the operator.  For
       example, in

           @ary = (1, 3, sort 4, 2);
           print @ary;         # prints 1324

       the commas on the right of the sort are evaluated before
       the sort, but the commas on the left are evaluated after.
       In other words, list operators tend to gobble up all
       arguments that follow, and then act like a simple TERM
       with regard to the preceding expression.  Be careful with
       parentheses:

           # These evaluate exit before doing the print:
           print($foo, exit);  # Obviously not what you want.
           print $foo, exit;   # Nor is this.

           # These do the print before evaluating exit:
           (print $foo), exit; # This is what you want.
           print($foo), exit;  # Or this.
           print ($foo), exit; # Or even this.

       Also note that

           print ($foo & 255) + 1, "\n";

       probably doesn't do what you expect at first glance.  The
       parentheses enclose the argument list for "print" which is
       evaluated (printing the result of "$foo & 255").  Then one
       is added to the return value of "print" (usually 1).  The



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       result is something like this:

           1 + 1, "\n";    # Obviously not what you meant.

       To do what you meant properly, you must write:

           print(($foo & 255) + 1, "\n");

       See "Named Unary Operators" for more discussion of this.

       Also parsed as terms are the "do {}" and "eval {}"
       constructs, as well as subroutine and method calls, and
       the anonymous constructors "[]" and "{}".

       See also "Quote and Quote-like Operators" toward the end
       of this section, as well as "I/O Operators".

   The Arrow Operator
       ""->"" is an infix dereference operator, just as it is in
       C and C++.  If the right side is either a "[...]",
       "{...}", or a "(...)" subscript, then the left side must
       be either a hard or symbolic reference to an array, a
       hash, or a subroutine respectively.  (Or technically
       speaking, a location capable of holding a hard reference,
       if it's an array or hash reference being used for
       assignment.)  See perlreftut and perlref.

       Otherwise, the right side is a method name or a simple
       scalar variable containing either the method name or a
       subroutine reference, and the left side must be either an
       object (a blessed reference) or a class name (that is, a
       package name).  See perlobj.

   Auto-increment and Auto-decrement
       "++" and "--" work as in C.  That is, if placed before a
       variable, they increment or decrement the variable by one
       before returning the value, and if placed after, increment
       or decrement after returning the value.

           $i = 0;  $j = 0;
           print $i++;  # prints 0
           print ++$j;  # prints 1

       Note that just as in C, Perl doesn't define when the
       variable is incremented or decremented. You just know it
       will be done sometime before or after the value is
       returned. This also means that modifying a variable twice
       in the same statement will lead to undefined behaviour.
       Avoid statements like:

           $i = $i ++;
           print ++ $i + $i ++;

       Perl will not guarantee what the result of the above



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       statements is.

       The auto-increment operator has a little extra builtin
       magic to it.  If you increment a variable that is numeric,
       or that has ever been used in a numeric context, you get a
       normal increment.  If, however, the variable has been used
       in only string contexts since it was set, and has a value
       that is not the empty string and matches the pattern
       "/^[a-zA-Z]*[0-9]*\z/", the increment is done as a string,
       preserving each character within its range, with carry:

           print ++($foo = '99');      # prints '100'
           print ++($foo = 'a0');      # prints 'a1'
           print ++($foo = 'Az');      # prints 'Ba'
           print ++($foo = 'zz');      # prints 'aaa'

       "undef" is always treated as numeric, and in particular is
       changed to 0 before incrementing (so that a post-increment
       of an undef value will return 0 rather than "undef").

       The auto-decrement operator is not magical.

   Exponentiation
       Binary "**" is the exponentiation operator.  It binds even
       more tightly than unary minus, so -2**4 is -(2**4), not
       (-2)**4. (This is implemented using C's pow(3) function,
       which actually works on doubles internally.)

   Symbolic Unary Operators
       Unary "!" performs logical negation, i.e., "not".  See
       also "not" for a lower precedence version of this.

       Unary "-" performs arithmetic negation if the operand is
       numeric.  If the operand is an identifier, a string
       consisting of a minus sign concatenated with the
       identifier is returned.  Otherwise, if the string starts
       with a plus or minus, a string starting with the opposite
       sign is returned.  One effect of these rules is that
       -bareword is equivalent to the string "-bareword".  If,
       however, the string begins with a non-alphabetic character
       (excluding "+" or "-"), Perl will attempt to convert the
       string to a numeric and the arithmetic negation is
       performed. If the string cannot be cleanly converted to a
       numeric, Perl will give the warning Argument "the string"
       isn't numeric in negation (-) at ....

       Unary "~" performs bitwise negation, i.e., 1's complement.
       For example, "0666 & ~027" is 0640.  (See also "Integer
       Arithmetic" and "Bitwise String Operators".)  Note that
       the width of the result is platform-dependent: ~0 is 32
       bits wide on a 32-bit platform, but 64 bits wide on a
       64-bit platform, so if you are expecting a certain bit
       width, remember to use the & operator to mask off the
       excess bits.



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       Unary "+" has no effect whatsoever, even on strings.  It
       is useful syntactically for separating a function name
       from a parenthesized expression that would otherwise be
       interpreted as the complete list of function arguments.
       (See examples above under "Terms and List Operators
       (Leftward)".)

       Unary "\" creates a reference to whatever follows it.  See
       perlreftut and perlref.  Do not confuse this behavior with
       the behavior of backslash within a string, although both
       forms do convey the notion of protecting the next thing
       from interpolation.

   Binding Operators
       Binary "=~" binds a scalar expression to a pattern match.
       Certain operations search or modify the string $_ by
       default.  This operator makes that kind of operation work
       on some other string.  The right argument is a search
       pattern, substitution, or transliteration.  The left
       argument is what is supposed to be searched, substituted,
       or transliterated instead of the default $_.  When used in
       scalar context, the return value generally indicates the
       success of the operation.  Behavior in list context
       depends on the particular operator.  See "Regexp Quote-
       Like Operators" for details and perlretut for examples
       using these operators.

       If the right argument is an expression rather than a
       search pattern, substitution, or transliteration, it is
       interpreted as a search pattern at run time. Note that
       this means that its contents will be interpolated twice,
       so

         '\\' =~ q'\\';

       is not ok, as the regex engine will end up trying to
       compile the pattern "\", which it will consider a syntax
       error.

       Binary "!~" is just like "=~" except the return value is
       negated in the logical sense.

   Multiplicative Operators
       Binary "*" multiplies two numbers.

       Binary "/" divides two numbers.

       Binary "%" is the modulo operator, which computes the
       division remainder of its first argument with respect to
       its second argument.  Given integer operands $a and $b: If
       $b is positive, then "$a % $b" is $a minus the largest
       multiple of $b less than or equal to $a.  If $b is
       negative, then "$a % $b" is $a minus the smallest multiple
       of $b that is not less than $a (i.e. the result will be



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       less than or equal to zero).  If the operands $a and $b
       are floating point values and the absolute value of $b
       (that is "abs($b)") is less than "(UV_MAX + 1)", only the
       integer portion of $a and $b will be used in the operation
       (Note: here "UV_MAX" means the maximum of the unsigned
       integer type).  If the absolute value of the right operand
       ("abs($b)") is greater than or equal to "(UV_MAX + 1)",
       "%" computes the floating-point remainder $r in the
       equation "($r = $a - $i*$b)" where $i is a certain integer
       that makes $r have the same sign as the right operand $b
       (not as the left operand $a like C function "fmod()") and
       the absolute value less than that of $b.  Note that when
       "use integer" is in scope, "%" gives you direct access to
       the modulo operator as implemented by your C compiler.
       This operator is not as well defined for negative
       operands, but it will execute faster.

       Binary "x" is the repetition operator.  In scalar context
       or if the left operand is not enclosed in parentheses, it
       returns a string consisting of the left operand repeated
       the number of times specified by the right operand.  In
       list context, if the left operand is enclosed in
       parentheses or is a list formed by "qw/STRING/", it
       repeats the list.  If the right operand is zero or
       negative, it returns an empty string or an empty list,
       depending on the context.

           print '-' x 80;             # print row of dashes

           print "\t" x ($tab/8), ' ' x ($tab%8);      # tab over

           @ones = (1) x 80;           # a list of 80 1's
           @ones = (5) x @ones;        # set all elements to 5

   Additive Operators
       Binary "+" returns the sum of two numbers.

       Binary "-" returns the difference of two numbers.

       Binary "." concatenates two strings.

   Shift Operators
       Binary "<<" returns the value of its left argument shifted
       left by the number of bits specified by the right
       argument.  Arguments should be integers.  (See also
       "Integer Arithmetic".)

       Binary ">>" returns the value of its left argument shifted
       right by the number of bits specified by the right
       argument.  Arguments should be integers.  (See also
       "Integer Arithmetic".)

       Note that both "<<" and ">>" in Perl are implemented
       directly using "<<" and ">>" in C.  If "use integer" (see



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       "Integer Arithmetic") is in force then signed C integers
       are used, else unsigned C integers are used.  Either way,
       the implementation isn't going to generate results larger
       than the size of the integer type Perl was built with (32
       bits or 64 bits).

       The result of overflowing the range of the integers is
       undefined because it is undefined also in C.  In other
       words, using 32-bit integers, "1 << 32" is undefined.
       Shifting by a negative number of bits is also undefined.

   Named Unary Operators
       The various named unary operators are treated as functions
       with one argument, with optional parentheses.

       If any list operator (print(), etc.) or any unary operator
       (chdir(), etc.)  is followed by a left parenthesis as the
       next token, the operator and arguments within parentheses
       are taken to be of highest precedence, just like a normal
       function call.  For example, because named unary operators
       are higher precedence than ||:

           chdir $foo    || die;       # (chdir $foo) || die
           chdir($foo)   || die;       # (chdir $foo) || die
           chdir ($foo)  || die;       # (chdir $foo) || die
           chdir +($foo) || die;       # (chdir $foo) || die

       but, because * is higher precedence than named operators:

           chdir $foo * 20;    # chdir ($foo * 20)
           chdir($foo) * 20;   # (chdir $foo) * 20
           chdir ($foo) * 20;  # (chdir $foo) * 20
           chdir +($foo) * 20; # chdir ($foo * 20)

           rand 10 * 20;       # rand (10 * 20)
           rand(10) * 20;      # (rand 10) * 20
           rand (10) * 20;     # (rand 10) * 20
           rand +(10) * 20;    # rand (10 * 20)

       Regarding precedence, the filetest operators, like "-f",
       "-M", etc. are treated like named unary operators, but
       they don't follow this functional parenthesis rule.  That
       means, for example, that "-f($file).".bak"" is equivalent
       to "-f "$file.bak"".

       See also "Terms and List Operators (Leftward)".

   Relational Operators
       Binary "<" returns true if the left argument is
       numerically less than the right argument.

       Binary ">" returns true if the left argument is
       numerically greater than the right argument.




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       Binary "<=" returns true if the left argument is
       numerically less than or equal to the right argument.

       Binary ">=" returns true if the left argument is
       numerically greater than or equal to the right argument.

       Binary "lt" returns true if the left argument is
       stringwise less than the right argument.

       Binary "gt" returns true if the left argument is
       stringwise greater than the right argument.

       Binary "le" returns true if the left argument is
       stringwise less than or equal to the right argument.

       Binary "ge" returns true if the left argument is
       stringwise greater than or equal to the right argument.

   Equality Operators
       Binary "==" returns true if the left argument is
       numerically equal to the right argument.

       Binary "!=" returns true if the left argument is
       numerically not equal to the right argument.

       Binary "<=>" returns -1, 0, or 1 depending on whether the
       left argument is numerically less than, equal to, or
       greater than the right argument.  If your platform
       supports NaNs (not-a-numbers) as numeric values, using
       them with "<=>" returns undef.  NaN is not "<", "==", ">",
       "<=" or ">=" anything (even NaN), so those 5 return false.
       NaN != NaN returns true, as does NaN != anything else. If
       your platform doesn't support NaNs then NaN is just a
       string with numeric value 0.

           perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
           perl -le '$a = "NaN"; print "NaN support here" if $a != $a'

       Binary "eq" returns true if the left argument is
       stringwise equal to the right argument.

       Binary "ne" returns true if the left argument is
       stringwise not equal to the right argument.

       Binary "cmp" returns -1, 0, or 1 depending on whether the
       left argument is stringwise less than, equal to, or
       greater than the right argument.

       Binary "~~" does a smart match between its arguments.
       Smart matching is described in "Smart matching in detail"
       in perlsyn.

       "lt", "le", "ge", "gt" and "cmp" use the collation (sort)
       order specified by the current locale if "use locale" is



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       in effect.  See perllocale.

   Bitwise And
       Binary "&" returns its operands ANDed together bit by bit.
       (See also "Integer Arithmetic" and "Bitwise String
       Operators".)

       Note that "&" has lower priority than relational
       operators, so for example the brackets are essential in a
       test like

               print "Even\n" if ($x & 1) == 0;

   Bitwise Or and Exclusive Or
       Binary "|" returns its operands ORed together bit by bit.
       (See also "Integer Arithmetic" and "Bitwise String
       Operators".)

       Binary "^" returns its operands XORed together bit by bit.
       (See also "Integer Arithmetic" and "Bitwise String
       Operators".)

       Note that "|" and "^" have lower priority than relational
       operators, so for example the brackets are essential in a
       test like

               print "false\n" if (8 | 2) != 10;

   C-style Logical And
       Binary "&&" performs a short-circuit logical AND
       operation.  That is, if the left operand is false, the
       right operand is not even evaluated.  Scalar or list
       context propagates down to the right operand if it is
       evaluated.

   C-style Logical Or
       Binary "||" performs a short-circuit logical OR operation.
       That is, if the left operand is true, the right operand is
       not even evaluated.  Scalar or list context propagates
       down to the right operand if it is evaluated.

   C-style Logical Defined-Or
       Although it has no direct equivalent in C, Perl's "//"
       operator is related to its C-style or.  In fact, it's
       exactly the same as "||", except that it tests the left
       hand side's definedness instead of its truth.  Thus, "$a
       // $b" is similar to "defined($a) || $b" (except that it
       returns the value of $a rather than the value of
       "defined($a)") and is exactly equivalent to "defined($a) ?
       $a : $b".  This is very useful for providing default
       values for variables.  If you actually want to test if at
       least one of $a and $b is defined, use "defined($a //
       $b)".




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       The "||", "//" and "&&" operators return the last value
       evaluated (unlike C's "||" and "&&", which return 0 or 1).
       Thus, a reasonably portable way to find out the home
       directory might be:

           $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
               (getpwuid($<))[7] // die "You're homeless!\n";

       In particular, this means that you shouldn't use this for
       selecting between two aggregates for assignment:

           @a = @b || @c;              # this is wrong
           @a = scalar(@b) || @c;      # really meant this
           @a = @b ? @b : @c;          # this works fine, though

       As more readable alternatives to "&&" and "||" when used
       for control flow, Perl provides the "and" and "or"
       operators (see below).  The short-circuit behavior is
       identical.  The precedence of "and" and "or" is much
       lower, however, so that you can safely use them after a
       list operator without the need for parentheses:

           unlink "alpha", "beta", "gamma"
                   or gripe(), next LINE;

       With the C-style operators that would have been written
       like this:

           unlink("alpha", "beta", "gamma")
                   || (gripe(), next LINE);

       Using "or" for assignment is unlikely to do what you want;
       see below.

   Range Operators
       Binary ".." is the range operator, which is really two
       different operators depending on the context.  In list
       context, it returns a list of values counting (up by ones)
       from the left value to the right value.  If the left value
       is greater than the right value then it returns the empty
       list.  The range operator is useful for writing "foreach
       (1..10)" loops and for doing slice operations on arrays.
       In the current implementation, no temporary array is
       created when the range operator is used as the expression
       in "foreach" loops, but older versions of Perl might burn
       a lot of memory when you write something like this:

           for (1 .. 1_000_000) {
               # code
           }

       The range operator also works on strings, using the
       magical auto-increment, see below.




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       In scalar context, ".." returns a boolean value.  The
       operator is bistable, like a flip-flop, and emulates the
       line-range (comma) operator of sed, awk, and various
       editors.  Each ".." operator maintains its own boolean
       state.  It is false as long as its left operand is false.
       Once the left operand is true, the range operator stays
       true until the right operand is true, AFTER which the
       range operator becomes false again.  It doesn't become
       false till the next time the range operator is evaluated.
       It can test the right operand and become false on the same
       evaluation it became true (as in awk), but it still
       returns true once.  If you don't want it to test the right
       operand till the next evaluation, as in sed, just use
       three dots ("...") instead of two.  In all other regards,
       "..." behaves just like ".." does.

       The right operand is not evaluated while the operator is
       in the "false" state, and the left operand is not
       evaluated while the operator is in the "true" state.  The
       precedence is a little lower than || and &&.  The value
       returned is either the empty string for false, or a
       sequence number (beginning with 1) for true.  The sequence
       number is reset for each range encountered.  The final
       sequence number in a range has the string "E0" appended to
       it, which doesn't affect its numeric value, but gives you
       something to search for if you want to exclude the
       endpoint.  You can exclude the beginning point by waiting
       for the sequence number to be greater than 1.

       If either operand of scalar ".." is a constant expression,
       that operand is considered true if it is equal ("==") to
       the current input line number (the $. variable).

       To be pedantic, the comparison is actually "int(EXPR) ==
       int(EXPR)", but that is only an issue if you use a
       floating point expression; when implicitly using $. as
       described in the previous paragraph, the comparison is
       "int(EXPR) == int($.)" which is only an issue when $.  is
       set to a floating point value and you are not reading from
       a file.  Furthermore, "span" .. "spat" or "2.18 .. 3.14"
       will not do what you want in scalar context because each
       of the operands are evaluated using their integer
       representation.

       Examples:

       As a scalar operator:

           if (101 .. 200) { print; } # print 2nd hundred lines, short for
                                      #   if ($. == 101 .. $. == 200) { print; }

           next LINE if (1 .. /^$/);  # skip header lines, short for
                                      #   next LINE if ($. == 1 .. /^$/);
                                      # (typically in a loop labeled LINE)



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           s/^/> / if (/^$/ .. eof());  # quote body

           # parse mail messages
           while (<>) {
               $in_header =   1  .. /^$/;
               $in_body   = /^$/ .. eof;
               if ($in_header) {
                   # do something
               } else { # in body
                   # do something else
               }
           } continue {
               close ARGV if eof;             # reset $. each file
           }

       Here's a simple example to illustrate the difference
       between the two range operators:

           @lines = ("   - Foo",
                     "01 - Bar",
                     "1  - Baz",
                     "   - Quux");

           foreach (@lines) {
               if (/0/ .. /1/) {
                   print "$_\n";
               }
           }

       This program will print only the line containing "Bar". If
       the range operator is changed to "...", it will also print
       the "Baz" line.

       And now some examples as a list operator:

           for (101 .. 200) { print; } # print $_ 100 times
           @foo = @foo[0 .. $#foo];    # an expensive no-op
           @foo = @foo[$#foo-4 .. $#foo];      # slice last 5 items

       The range operator (in list context) makes use of the
       magical auto-increment algorithm if the operands are
       strings.  You can say

           @alphabet = ('A' .. 'Z');

       to get all normal letters of the English alphabet, or

           $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];

       to get a hexadecimal digit, or

           @z2 = ('01' .. '31');  print $z2[$mday];




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       to get dates with leading zeros.

       If the final value specified is not in the sequence that
       the magical increment would produce, the sequence goes
       until the next value would be longer than the final value
       specified.

       If the initial value specified isn't part of a magical
       increment sequence (that is, a non-empty string matching
       "/^[a-zA-Z]*[0-9]*\z/"), only the initial value will be
       returned.  So the following will only return an alpha:

           use charnames 'greek';
           my @greek_small =  ("\N{alpha}" .. "\N{omega}");

       To get lower-case greek letters, use this instead:

           my @greek_small =  map { chr } ( ord("\N{alpha}") .. ord("\N{omega}") );

       Because each operand is evaluated in integer form, "2.18
       .. 3.14" will return two elements in list context.

           @list = (2.18 .. 3.14); # same as @list = (2 .. 3);

   Conditional Operator
       Ternary "?:" is the conditional operator, just as in C.
       It works much like an if-then-else.  If the argument
       before the ? is true, the argument before the : is
       returned, otherwise the argument after the : is returned.
       For example:

           printf "I have %d dog%s.\n", $n,
                   ($n == 1) ? '' : "s";

       Scalar or list context propagates downward into the 2nd or
       3rd argument, whichever is selected.

           $a = $ok ? $b : $c;  # get a scalar
           @a = $ok ? @b : @c;  # get an array
           $a = $ok ? @b : @c;  # oops, that's just a count!

       The operator may be assigned to if both the 2nd and 3rd
       arguments are legal lvalues (meaning that you can assign
       to them):

           ($a_or_b ? $a : $b) = $c;

       Because this operator produces an assignable result, using
       assignments without parentheses will get you in trouble.
       For example, this:

           $a % 2 ? $a += 10 : $a += 2

       Really means this:



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           (($a % 2) ? ($a += 10) : $a) += 2

       Rather than this:

           ($a % 2) ? ($a += 10) : ($a += 2)

       That should probably be written more simply as:

           $a += ($a % 2) ? 10 : 2;

   Assignment Operators
       "=" is the ordinary assignment operator.

       Assignment operators work as in C.  That is,

           $a += 2;

       is equivalent to

           $a = $a + 2;

       although without duplicating any side effects that
       dereferencing the lvalue might trigger, such as from
       tie().  Other assignment operators work similarly.  The
       following are recognized:

           **=    +=    *=    &=    <<=    &&=
                  -=    /=    |=    >>=    ||=
                  .=    %=    ^=           //=
                        x=

       Although these are grouped by family, they all have the
       precedence of assignment.

       Unlike in C, the scalar assignment operator produces a
       valid lvalue.  Modifying an assignment is equivalent to
       doing the assignment and then modifying the variable that
       was assigned to.  This is useful for modifying a copy of
       something, like this:

           ($tmp = $global) =~ tr [A-Z] [a-z];

       Likewise,

           ($a += 2) *= 3;

       is equivalent to

           $a += 2;
           $a *= 3;

       Similarly, a list assignment in list context produces the
       list of lvalues assigned to, and a list assignment in
       scalar context returns the number of elements produced by



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       the expression on the right hand side of the assignment.

   Comma Operator
       Binary "," is the comma operator.  In scalar context it
       evaluates its left argument, throws that value away, then
       evaluates its right argument and returns that value.  This
       is just like C's comma operator.

       In list context, it's just the list argument separator,
       and inserts both its arguments into the list.  These
       arguments are also evaluated from left to right.

       The "=>" operator is a synonym for the comma except that
       it causes its left operand to be interpreted as a string
       if it begins with a letter or underscore and is composed
       only of letters, digits and underscores.  This includes
       operands that might otherwise be interpreted as operators,
       constants, single number v-strings or function calls. If
       in doubt about this behaviour, the left operand can be
       quoted explicitly.

       Otherwise, the "=>" operator behaves exactly as the comma
       operator or list argument separator, according to context.

       For example:

           use constant FOO => "something";

           my %h = ( FOO => 23 );

       is equivalent to:

           my %h = ("FOO", 23);

       It is NOT:

           my %h = ("something", 23);

       The "=>" operator is helpful in documenting the
       correspondence between keys and values in hashes, and
       other paired elements in lists.

               %hash = ( $key => $value );
               login( $username => $password );

   List Operators (Rightward)
       On the right side of a list operator, it has very low
       precedence, such that it controls all comma-separated
       expressions found there.  The only operators with lower
       precedence are the logical operators "and", "or", and
       "not", which may be used to evaluate calls to list
       operators without the need for extra parentheses:





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           open HANDLE, "filename"
               or die "Can't open: $!\n";

       See also discussion of list operators in "Terms and List
       Operators (Leftward)".

   Logical Not
       Unary "not" returns the logical negation of the expression
       to its right.  It's the equivalent of "!" except for the
       very low precedence.

   Logical And
       Binary "and" returns the logical conjunction of the two
       surrounding expressions.  It's equivalent to && except for
       the very low precedence.  This means that it short-
       circuits: i.e., the right expression is evaluated only if
       the left expression is true.

   Logical or, Defined or, and Exclusive Or
       Binary "or" returns the logical disjunction of the two
       surrounding expressions.  It's equivalent to || except for
       the very low precedence.  This makes it useful for control
       flow

           print FH $data              or die "Can't write to FH: $!";

       This means that it short-circuits: i.e., the right
       expression is evaluated only if the left expression is
       false.  Due to its precedence, you should probably avoid
       using this for assignment, only for control flow.

           $a = $b or $c;              # bug: this is wrong
           ($a = $b) or $c;            # really means this
           $a = $b || $c;              # better written this way

       However, when it's a list-context assignment and you're
       trying to use "||" for control flow, you probably need
       "or" so that the assignment takes higher precedence.

           @info = stat($file) || die;     # oops, scalar sense of stat!
           @info = stat($file) or die;     # better, now @info gets its due

       Then again, you could always use parentheses.

       Binary "xor" returns the exclusive-OR of the two
       surrounding expressions.  It cannot short circuit, of
       course.

   C Operators Missing From Perl
       Here is what C has that Perl doesn't:

       unary & Address-of operator.  (But see the "\" operator
               for taking a reference.)




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       unary * Dereference-address operator. (Perl's prefix
               dereferencing operators are typed: $, @, %, and
               &.)

       (TYPE)  Type-casting operator.

   Quote and Quote-like Operators
       While we usually think of quotes as literal values, in
       Perl they function as operators, providing various kinds
       of interpolating and pattern matching capabilities.  Perl
       provides customary quote characters for these behaviors,
       but also provides a way for you to choose your quote
       character for any of them.  In the following table, a "{}"
       represents any pair of delimiters you choose.

           Customary  Generic        Meaning        Interpolates
               ''       q{}          Literal             no
               ""      qq{}          Literal             yes
               ``      qx{}          Command             yes*
                       qw{}         Word list            no
               //       m{}       Pattern match          yes*
                       qr{}          Pattern             yes*
                        s{}{}      Substitution          yes*
                       tr{}{}    Transliteration         no (but see below)
               <<EOF                 here-doc            yes*

               * unless the delimiter is ''.

       Non-bracketing delimiters use the same character fore and
       aft, but the four sorts of brackets (round, angle, square,
       curly) will all nest, which means that

               q{foo{bar}baz}

       is the same as

               'foo{bar}baz'

       Note, however, that this does not always work for quoting
       Perl code:

               $s = q{ if($a eq "}") ... }; # WRONG

       is a syntax error. The "Text::Balanced" module (from CPAN,
       and starting from Perl 5.8 part of the standard
       distribution) is able to do this properly.

       There can be whitespace between the operator and the
       quoting characters, except when "#" is being used as the
       quoting character.  "q#foo#" is parsed as the string
       "foo", while "q #foo#" is the operator "q" followed by a
       comment.  Its argument will be taken from the next line.
       This allows you to write:




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           s {foo}  # Replace foo
             {bar}  # with bar.

       The following escape sequences are available in constructs
       that interpolate and in transliterations.

           \t          tab             (HT, TAB)
           \n          newline         (NL)
           \r          return          (CR)
           \f          form feed       (FF)
           \b          backspace       (BS)
           \a          alarm (bell)    (BEL)
           \e          escape          (ESC)
           \033        octal char      (example: ESC)
           \x1b        hex char        (example: ESC)
           \x{263a}    wide hex char   (example: SMILEY)
           \c[         control char    (example: ESC)
           \N{name}    named Unicode character

       The character following "\c" is mapped to some other
       character by converting letters to upper case and then (on
       ASCII systems) by inverting the 7th bit (0x40). The most
       interesting range is from '@' to '_' (0x40 through 0x5F),
       resulting in a control character from 0x00 through 0x1F. A
       '?' maps to the DEL character. On EBCDIC systems only '@',
       the letters, '[', '\', ']', '^', '_' and '?' will work,
       resulting in 0x00 through 0x1F and 0x7F.

       NOTE: Unlike C and other languages, Perl has no \v escape
       sequence for the vertical tab (VT - ASCII 11), but you may
       use "\ck" or "\x0b".

       The following escape sequences are available in constructs
       that interpolate but not in transliterations.

           \l          lowercase next char
           \u          uppercase next char
           \L          lowercase till \E
           \U          uppercase till \E
           \E          end case modification
           \Q          quote non-word characters till \E

       If "use locale" is in effect, the case map used by "\l",
       "\L", "\u" and "\U" is taken from the current locale.  See
       perllocale.  If Unicode (for example, "\N{}" or wide hex
       characters of 0x100 or beyond) is being used, the case map
       used by "\l", "\L", "\u" and "\U" is as defined by
       Unicode.  For documentation of "\N{name}", see charnames.

       All systems use the virtual "\n" to represent a line
       terminator, called a "newline".  There is no such thing as
       an unvarying, physical newline character.  It is only an
       illusion that the operating system, device drivers, C
       libraries, and Perl all conspire to preserve.  Not all



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       systems read "\r" as ASCII CR and "\n" as ASCII LF.  For
       example, on a Mac, these are reversed, and on systems
       without line terminator, printing "\n" may emit no actual
       data.  In general, use "\n" when you mean a "newline" for
       your system, but use the literal ASCII when you need an
       exact character.  For example, most networking protocols
       expect and prefer a CR+LF ("\015\012" or "\cM\cJ") for
       line terminators, and although they often accept just
       "\012", they seldom tolerate just "\015".  If you get in
       the habit of using "\n" for networking, you may be burned
       some day.

       For constructs that do interpolate, variables beginning
       with ""$"" or ""@"" are interpolated.  Subscripted
       variables such as $a[3] or "$href->{key}[0]" are also
       interpolated, as are array and hash slices.  But method
       calls such as "$obj->meth" are not.

       Interpolating an array or slice interpolates the elements
       in order, separated by the value of $", so is equivalent
       to interpolating "join $", @array".    "Punctuation"
       arrays such as "@*" are only interpolated if the name is
       enclosed in braces "@{*}", but special arrays @_, "@+",
       and "@-" are interpolated, even without braces.

       You cannot include a literal "$" or "@" within a "\Q"
       sequence.  An unescaped "$" or "@" interpolates the
       corresponding variable, while escaping will cause the
       literal string "\$" to be inserted.  You'll need to write
       something like "m/\Quser\E\@\Qhost/".

       Patterns are subject to an additional level of
       interpretation as a regular expression.  This is done as a
       second pass, after variables are interpolated, so that
       regular expressions may be incorporated into the pattern
       from the variables.  If this is not what you want, use
       "\Q" to interpolate a variable literally.

       Apart from the behavior described above, Perl does not
       expand multiple levels of interpolation.  In particular,
       contrary to the expectations of shell programmers, back-
       quotes do NOT interpolate within double quotes, nor do
       single quotes impede evaluation of variables when used
       within double quotes.

   Regexp Quote-Like Operators
       Here are the quote-like operators that apply to pattern
       matching and related activities.

       qr/STRING/msixpo
               This operator quotes (and possibly compiles) its
               STRING as a regular expression.  STRING is
               interpolated the same way as PATTERN in
               "m/PATTERN/".  If "'" is used as the delimiter, no



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               interpolation is done.  Returns a Perl value which
               may be used instead of the corresponding
               "/STRING/msixpo" expression. The returned value is
               a normalized version of the original pattern. It
               magically differs from a string containing the
               same characters: "ref(qr/x/)" returns "Regexp",
               even though dereferencing the result returns
               undef.

               For example,

                   $rex = qr/my.STRING/is;
                   print $rex;                 # prints (?si-xm:my.STRING)
                   s/$rex/foo/;

               is equivalent to

                   s/my.STRING/foo/is;

               The result may be used as a subpattern in a match:

                   $re = qr/$pattern/;
                   $string =~ /foo${re}bar/;   # can be interpolated in other patterns
                   $string =~ $re;             # or used standalone
                   $string =~ /$re/;           # or this way

               Since Perl may compile the pattern at the moment
               of execution of qr() operator, using qr() may have
               speed advantages in some situations, notably if
               the result of qr() is used standalone:

                   sub match {
                       my $patterns = shift;
                       my @compiled = map qr/$_/i, @$patterns;
                       grep {
                           my $success = 0;
                           foreach my $pat (@compiled) {
                               $success = 1, last if /$pat/;
                           }
                           $success;
                       } @_;
                   }

               Precompilation of the pattern into an internal
               representation at the moment of qr() avoids a need
               to recompile the pattern every time a match
               "/$pat/" is attempted.  (Perl has many other
               internal optimizations, but none would be
               triggered in the above example if we did not use
               qr() operator.)

               Options are:





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                   m   Treat string as multiple lines.
                   s   Treat string as single line. (Make . match a newline)
                   i   Do case-insensitive pattern matching.
                   x   Use extended regular expressions.
                   p   When matching preserve a copy of the matched string so
                       that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
                   o   Compile pattern only once.

               If a precompiled pattern is embedded in a larger
               pattern then the effect of 'msixp' will be
               propagated appropriately.  The effect of the 'o'
               modifier has is not propagated, being restricted
               to those patterns explicitly using it.

               See perlre for additional information on valid
               syntax for STRING, and for a detailed look at the
               semantics of regular expressions.

       m/PATTERN/msixpogc
       /PATTERN/msixpogc
               Searches a string for a pattern match, and in
               scalar context returns true if it succeeds, false
               if it fails.  If no string is specified via the
               "=~" or "!~" operator, the $_ string is searched.
               (The string specified with "=~" need not be an
               lvalue--it may be the result of an expression
               evaluation, but remember the "=~" binds rather
               tightly.)  See also perlre.  See perllocale for
               discussion of additional considerations that apply
               when "use locale" is in effect.

               Options are as described in "qr//"; in addition,
               the following match process modifiers are
               available:

                   g   Match globally, i.e., find all occurrences.
                   c   Do not reset search position on a failed match when /g is in effect.

               If "/" is the delimiter then the initial "m" is
               optional.  With the "m" you can use any pair of
               non-alphanumeric, non-whitespace characters as
               delimiters.  This is particularly useful for
               matching path names that contain "/", to avoid LTS
               (leaning toothpick syndrome).  If "?" is the
               delimiter, then the match-only-once rule of
               "?PATTERN?" applies.  If "'" is the delimiter, no
               interpolation is performed on the PATTERN.

               PATTERN may contain variables, which will be
               interpolated (and the pattern recompiled) every
               time the pattern search is evaluated, except for
               when the delimiter is a single quote.  (Note that
               $(, $), and $| are not interpolated because they
               look like end-of-string tests.)  If you want such



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               a pattern to be compiled only once, add a "/o"
               after the trailing delimiter.  This avoids
               expensive run-time recompilations, and is useful
               when the value you are interpolating won't change
               over the life of the script.  However, mentioning
               "/o" constitutes a promise that you won't change
               the variables in the pattern.  If you change them,
               Perl won't even notice.  See also "STRING/msixpo""
               in "qr.

       The empty pattern //
               If the PATTERN evaluates to the empty string, the
               last successfully matched regular expression is
               used instead. In this case, only the "g" and "c"
               flags on the empty pattern is honoured - the other
               flags are taken from the original pattern. If no
               match has previously succeeded, this will
               (silently) act instead as a genuine empty pattern
               (which will always match).

               Note that it's possible to confuse Perl into
               thinking "//" (the empty regex) is really "//"
               (the defined-or operator).  Perl is usually pretty
               good about this, but some pathological cases might
               trigger this, such as "$a///" (is that "($a) /
               (//)" or "$a // /"?) and "print $fh //" ("print
               $fh(//" or "print($fh //"?).  In all of these
               examples, Perl will assume you meant defined-or.
               If you meant the empty regex, just use parentheses
               or spaces to disambiguate, or even prefix the
               empty regex with an "m" (so "//" becomes "m//").

       Matching in list context
               If the "/g" option is not used, "m//" in list
               context returns a list consisting of the
               subexpressions matched by the parentheses in the
               pattern, i.e., ($1, $2, $3...).  (Note that here
               $1 etc. are also set, and that this differs from
               Perl 4's behavior.)  When there are no parentheses
               in the pattern, the return value is the list "(1)"
               for success.  With or without parentheses, an
               empty list is returned upon failure.

               Examples:

                   open(TTY, '/dev/tty');
                   <TTY> =~ /^y/i && foo();    # do foo if desired

                   if (/Version: *([0-9.]*)/) { $version = $1; }

                   next if m#^/usr/spool/uucp#;

                   # poor man's grep
                   $arg = shift;



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                   while (<>) {
                       print if /$arg/o;       # compile only once
                   }

                   if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))

               This last example splits $foo into the first two
               words and the remainder of the line, and assigns
               those three fields to $F1, $F2, and $Etc.  The
               conditional is true if any variables were
               assigned, i.e., if the pattern matched.

               The "/g" modifier specifies global pattern
               matching--that is, matching as many times as
               possible within the string.  How it behaves
               depends on the context.  In list context, it
               returns a list of the substrings matched by any
               capturing parentheses in the regular expression.
               If there are no parentheses, it returns a list of
               all the matched strings, as if there were
               parentheses around the whole pattern.

               In scalar context, each execution of "m//g" finds
               the next match, returning true if it matches, and
               false if there is no further match.  The position
               after the last match can be read or set using the
               pos() function; see "pos" in perlfunc.   A failed
               match normally resets the search position to the
               beginning of the string, but you can avoid that by
               adding the "/c" modifier (e.g. "m//gc").
               Modifying the target string also resets the search
               position.

       \G assertion
               You can intermix "m//g" matches with "m/\G.../g",
               where "\G" is a zero-width assertion that matches
               the exact position where the previous "m//g", if
               any, left off.  Without the "/g" modifier, the
               "\G" assertion still anchors at pos(), but the
               match is of course only attempted once.  Using
               "\G" without "/g" on a target string that has not
               previously had a "/g" match applied to it is the
               same as using the "\A" assertion to match the
               beginning of the string.  Note also that,
               currently, "\G" is only properly supported when
               anchored at the very beginning of the pattern.

               Examples:

                   # list context
                   ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);

                   # scalar context
                   $/ = "";



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                   while (defined($paragraph = <>)) {
                       while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
                           $sentences++;
                       }
                   }
                   print "$sentences\n";

                   # using m//gc with \G
                   $_ = "ppooqppqq";
                   while ($i++ < 2) {
                       print "1: '";
                       print $1 while /(o)/gc; print "', pos=", pos, "\n";
                       print "2: '";
                       print $1 if /\G(q)/gc;  print "', pos=", pos, "\n";
                       print "3: '";
                       print $1 while /(p)/gc; print "', pos=", pos, "\n";
                   }
                   print "Final: '$1', pos=",pos,"\n" if /\G(.)/;

               The last example should print:

                   1: 'oo', pos=4
                   2: 'q', pos=5
                   3: 'pp', pos=7
                   1: '', pos=7
                   2: 'q', pos=8
                   3: '', pos=8
                   Final: 'q', pos=8

               Notice that the final match matched "q" instead of
               "p", which a match without the "\G" anchor would
               have done. Also note that the final match did not
               update "pos" -- "pos" is only updated on a "/g"
               match. If the final match did indeed match "p",
               it's a good bet that you're running an older
               (pre-5.6.0) Perl.

               A useful idiom for "lex"-like scanners is
               "/\G.../gc".  You can combine several regexps like
               this to process a string part-by-part, doing
               different actions depending on which regexp
               matched.  Each regexp tries to match where the
               previous one leaves off.

                $_ = <<'EOL';
                     $url = URI::URL->new( "http://www/" );   die if $url eq "xXx";
                EOL
                LOOP:
                   {
                     print(" digits"),         redo LOOP if /\G\d+\b[,.;]?\s*/gc;
                     print(" lowercase"),      redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
                     print(" UPPERCASE"),      redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
                     print(" Capitalized"),    redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
                     print(" MiXeD"),          redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;



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                     print(" alphanumeric"),   redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
                     print(" line-noise"),     redo LOOP if /\G[^A-Za-z0-9]+/gc;
                     print ". That's all!\n";
                   }

               Here is the output (split into several lines):

                line-noise lowercase line-noise lowercase UPPERCASE line-noise
                UPPERCASE line-noise lowercase line-noise lowercase line-noise
                lowercase lowercase line-noise lowercase lowercase line-noise
                MiXeD line-noise. That's all!

       ?PATTERN?
               This is just like the "/pattern/" search, except
               that it matches only once between calls to the
               reset() operator.  This is a useful optimization
               when you want to see only the first occurrence of
               something in each file of a set of files, for
               instance.  Only "??"  patterns local to the
               current package are reset.

                   while (<>) {
                       if (?^$?) {
                                           # blank line between header and body
                       }
                   } continue {
                       reset if eof;       # clear ?? status for next file
                   }

               This usage is vaguely deprecated, which means it
               just might possibly be removed in some distant
               future version of Perl, perhaps somewhere around
               the year 2168.

       s/PATTERN/REPLACEMENT/msixpogce
               Searches a string for a pattern, and if found,
               replaces that pattern with the replacement text
               and returns the number of substitutions made.
               Otherwise it returns false (specifically, the
               empty string).

               If no string is specified via the "=~" or "!~"
               operator, the $_ variable is searched and
               modified.  (The string specified with "=~" must be
               scalar variable, an array element, a hash element,
               or an assignment to one of those, i.e., an
               lvalue.)

               If the delimiter chosen is a single quote, no
               interpolation is done on either the PATTERN or the
               REPLACEMENT.  Otherwise, if the PATTERN contains a
               $ that looks like a variable rather than an end-
               of-string test, the variable will be interpolated
               into the pattern at run-time.  If you want the



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               pattern compiled only once the first time the
               variable is interpolated, use the "/o" option.  If
               the pattern evaluates to the empty string, the
               last successfully executed regular expression is
               used instead.  See perlre for further explanation
               on these.  See perllocale for discussion of
               additional considerations that apply when "use
               locale" is in effect.

               Options are as with m// with the addition of the
               following replacement specific options:

                   e   Evaluate the right side as an expression.
                   ee  Evaluate the right side as a string then eval the result

               Any non-alphanumeric, non-whitespace delimiter may
               replace the slashes.  If single quotes are used,
               no interpretation is done on the replacement
               string (the "/e" modifier overrides this,
               however).  Unlike Perl 4, Perl 5 treats backticks
               as normal delimiters; the replacement text is not
               evaluated as a command.  If the PATTERN is
               delimited by bracketing quotes, the REPLACEMENT
               has its own pair of quotes, which may or may not
               be bracketing quotes, e.g., "s(foo)(bar)" or
               "s<foo>/bar/".  A "/e" will cause the replacement
               portion to be treated as a full-fledged Perl
               expression and evaluated right then and there.  It
               is, however, syntax checked at compile-time. A
               second "e" modifier will cause the replacement
               portion to be "eval"ed before being run as a Perl
               expression.

               Examples:

                   s/\bgreen\b/mauve/g;                # don't change wintergreen

                   $path =~ s|/usr/bin|/usr/local/bin|;

                   s/Login: $foo/Login: $bar/; # run-time pattern

                   ($foo = $bar) =~ s/this/that/;      # copy first, then change

                   $count = ($paragraph =~ s/Mister\b/Mr./g);  # get change-count

                   $_ = 'abc123xyz';
                   s/\d+/$&*2/e;               # yields 'abc246xyz'
                   s/\d+/sprintf("%5d",$&)/e;  # yields 'abc  246xyz'
                   s/\w/$& x 2/eg;             # yields 'aabbcc  224466xxyyzz'

                   s/%(.)/$percent{$1}/g;      # change percent escapes; no /e
                   s/%(.)/$percent{$1} || $&/ge;       # expr now, so /e
                   s/^=(\w+)/pod($1)/ge;       # use function call




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                   # expand variables in $_, but dynamics only, using
                   # symbolic dereferencing
                   s/\$(\w+)/${$1}/g;

                   # Add one to the value of any numbers in the string
                   s/(\d+)/1 + $1/eg;

                   # This will expand any embedded scalar variable
                   # (including lexicals) in $_ : First $1 is interpolated
                   # to the variable name, and then evaluated
                   s/(\$\w+)/$1/eeg;

                   # Delete (most) C comments.
                   $program =~ s {
                       /\*     # Match the opening delimiter.
                       .*?     # Match a minimal number of characters.
                       \*/     # Match the closing delimiter.
                   } []gsx;

                   s/^\s*(.*?)\s*$/$1/;        # trim whitespace in $_, expensively

                   for ($variable) {           # trim whitespace in $variable, cheap
                       s/^\s+//;
                       s/\s+$//;
                   }

                   s/([^ ]*) *([^ ]*)/$2 $1/;  # reverse 1st two fields

               Note the use of $ instead of \ in the last
               example.  Unlike sed, we use the \<digit> form in
               only the left hand side.  Anywhere else it's
               $<digit>.

               Occasionally, you can't use just a "/g" to get all
               the changes to occur that you might want.  Here
               are two common cases:

                   # put commas in the right places in an integer
                   1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;

                   # expand tabs to 8-column spacing
                   1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;

   Quote-Like Operators
       q/STRING/
       'STRING'
           A single-quoted, literal string.  A backslash
           represents a backslash unless followed by the
           delimiter or another backslash, in which case the
           delimiter or backslash is interpolated.

               $foo = q!I said, "You said, 'She said it.'"!;
               $bar = q('This is it.');
               $baz = '\n';                # a two-character string



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       qq/STRING/
       "STRING"
           A double-quoted, interpolated string.

               $_ .= qq
                (*** The previous line contains the naughty word "$1".\n)
                           if /\b(tcl|java|python)\b/i;      # :-)
               $baz = "\n";                # a one-character string

       qx/STRING/
       `STRING`
           A string which is (possibly) interpolated and then
           executed as a system command with "/bin/sh" or its
           equivalent.  Shell wildcards, pipes, and redirections
           will be honored.  The collected standard output of the
           command is returned; standard error is unaffected.  In
           scalar context, it comes back as a single (potentially
           multi-line) string, or undef if the command failed.
           In list context, returns a list of lines (however
           you've defined lines with $/ or
           $INPUT_RECORD_SEPARATOR), or an empty list if the
           command failed.

           Because backticks do not affect standard error, use
           shell file descriptor syntax (assuming the shell
           supports this) if you care to address this.  To
           capture a command's STDERR and STDOUT together:

               $output = `cmd 2>&1`;

           To capture a command's STDOUT but discard its STDERR:

               $output = `cmd 2>/dev/null`;

           To capture a command's STDERR but discard its STDOUT
           (ordering is important here):

               $output = `cmd 2>&1 1>/dev/null`;

           To exchange a command's STDOUT and STDERR in order to
           capture the STDERR but leave its STDOUT to come out
           the old STDERR:

               $output = `cmd 3>&1 1>&2 2>&3 3>&-`;

           To read both a command's STDOUT and its STDERR
           separately, it's easiest to redirect them separately
           to files, and then read from those files when the
           program is done:

               system("program args 1>program.stdout 2>program.stderr");

           The STDIN filehandle used by the command is inherited
           from Perl's STDIN.  For example:



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               open BLAM, "blam" || die "Can't open: $!";
               open STDIN, "<&BLAM";
               print `sort`;

           will print the sorted contents of the file "blam".

           Using single-quote as a delimiter protects the command
           from Perl's double-quote interpolation, passing it on
           to the shell instead:

               $perl_info  = qx(ps $$);            # that's Perl's $$
               $shell_info = qx'ps $$';            # that's the new shell's $$

           How that string gets evaluated is entirely subject to
           the command interpreter on your system.  On most
           platforms, you will have to protect shell
           metacharacters if you want them treated literally.
           This is in practice difficult to do, as it's unclear
           how to escape which characters.  See perlsec for a
           clean and safe example of a manual fork() and exec()
           to emulate backticks safely.

           On some platforms (notably DOS-like ones), the shell
           may not be capable of dealing with multiline commands,
           so putting newlines in the string may not get you what
           you want.  You may be able to evaluate multiple
           commands in a single line by separating them with the
           command separator character, if your shell supports
           that (e.g. ";" on many Unix shells; "&" on the Windows
           NT "cmd" shell).

           Beginning with v5.6.0, Perl will attempt to flush all
           files opened for output before starting the child
           process, but this may not be supported on some
           platforms (see perlport).  To be safe, you may need to
           set $| ($AUTOFLUSH in English) or call the
           "autoflush()" method of "IO::Handle" on any open
           handles.

           Beware that some command shells may place restrictions
           on the length of the command line.  You must ensure
           your strings don't exceed this limit after any
           necessary interpolations.  See the platform-specific
           release notes for more details about your particular
           environment.

           Using this operator can lead to programs that are
           difficult to port, because the shell commands called
           vary between systems, and may in fact not be present
           at all.  As one example, the "type" command under the
           POSIX shell is very different from the "type" command
           under DOS.  That doesn't mean you should go out of
           your way to avoid backticks when they're the right way
           to get something done.  Perl was made to be a glue



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           language, and one of the things it glues together is
           commands.  Just understand what you're getting
           yourself into.

           See "I/O Operators" for more discussion.

       qw/STRING/
           Evaluates to a list of the words extracted out of
           STRING, using embedded whitespace as the word
           delimiters.  It can be understood as being roughly
           equivalent to:

               split(' ', q/STRING/);

           the differences being that it generates a real list at
           compile time, and in scalar context it returns the
           last element in the list.  So this expression:

               qw(foo bar baz)

           is semantically equivalent to the list:

               'foo', 'bar', 'baz'

           Some frequently seen examples:

               use POSIX qw( setlocale localeconv )
               @EXPORT = qw( foo bar baz );

           A common mistake is to try to separate the words with
           comma or to put comments into a multi-line
           "qw"-string.  For this reason, the "use warnings"
           pragma and the -w switch (that is, the $^W variable)
           produces warnings if the STRING contains the "," or
           the "#" character.

       tr/SEARCHLIST/REPLACEMENTLIST/cds
       y/SEARCHLIST/REPLACEMENTLIST/cds
           Transliterates all occurrences of the characters found
           in the search list with the corresponding character in
           the replacement list.  It returns the number of
           characters replaced or deleted.  If no string is
           specified via the =~ or !~ operator, the $_ string is
           transliterated.  (The string specified with =~ must be
           a scalar variable, an array element, a hash element,
           or an assignment to one of those, i.e., an lvalue.)

           A character range may be specified with a hyphen, so
           "tr/A-J/0-9/" does the same replacement as
           "tr/ACEGIBDFHJ/0246813579/".  For sed devotees, "y" is
           provided as a synonym for "tr".  If the SEARCHLIST is
           delimited by bracketing quotes, the REPLACEMENTLIST
           has its own pair of quotes, which may or may not be
           bracketing quotes, e.g., "tr[A-Z][a-z]" or



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           "tr(+\-*/)/ABCD/".

           Note that "tr" does not do regular expression
           character classes such as "\d" or "[:lower:]".  The
           "tr" operator is not equivalent to the tr(1) utility.
           If you want to map strings between lower/upper cases,
           see "lc" in perlfunc and "uc" in perlfunc, and in
           general consider using the "s" operator if you need
           regular expressions.

           Note also that the whole range idea is rather
           unportable between character sets--and even within
           character sets they may cause results you probably
           didn't expect.  A sound principle is to use only
           ranges that begin from and end at either alphabets of
           equal case (a-e, A-E), or digits (0-4).  Anything else
           is unsafe.  If in doubt, spell out the character sets
           in full.

           Options:

               c   Complement the SEARCHLIST.
               d   Delete found but unreplaced characters.
               s   Squash duplicate replaced characters.

           If the "/c" modifier is specified, the SEARCHLIST
           character set is complemented.  If the "/d" modifier
           is specified, any characters specified by SEARCHLIST
           not found in REPLACEMENTLIST are deleted.  (Note that
           this is slightly more flexible than the behavior of
           some tr programs, which delete anything they find in
           the SEARCHLIST, period.) If the "/s" modifier is
           specified, sequences of characters that were
           transliterated to the same character are squashed down
           to a single instance of the character.

           If the "/d" modifier is used, the REPLACEMENTLIST is
           always interpreted exactly as specified.  Otherwise,
           if the REPLACEMENTLIST is shorter than the SEARCHLIST,
           the final character is replicated till it is long
           enough.  If the REPLACEMENTLIST is empty, the
           SEARCHLIST is replicated.  This latter is useful for
           counting characters in a class or for squashing
           character sequences in a class.

           Examples:

               $ARGV[1] =~ tr/A-Z/a-z/;    # canonicalize to lower case

               $cnt = tr/*/*/;             # count the stars in $_

               $cnt = $sky =~ tr/*/*/;     # count the stars in $sky

               $cnt = tr/0-9//;            # count the digits in $_



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               tr/a-zA-Z//s;               # bookkeeper -> bokeper

               ($HOST = $host) =~ tr/a-z/A-Z/;

               tr/a-zA-Z/ /cs;             # change non-alphas to single space

               tr [\200-\377]
                  [\000-\177];             # delete 8th bit

           If multiple transliterations are given for a
           character, only the first one is used:

               tr/AAA/XYZ/

           will transliterate any A to X.

           Because the transliteration table is built at compile
           time, neither the SEARCHLIST nor the REPLACEMENTLIST
           are subjected to double quote interpolation.  That
           means that if you want to use variables, you must use
           an eval():

               eval "tr/$oldlist/$newlist/";
               die $@ if $@;

               eval "tr/$oldlist/$newlist/, 1" or die $@;

       <<EOF
           A line-oriented form of quoting is based on the shell
           "here-document" syntax.  Following a "<<" you specify
           a string to terminate the quoted material, and all
           lines following the current line down to the
           terminating string are the value of the item.

           The terminating string may be either an identifier (a
           word), or some quoted text.  An unquoted identifier
           works like double quotes.  There may not be a space
           between the "<<" and the identifier, unless the
           identifier is explicitly quoted.  (If you put a space
           it will be treated as a null identifier, which is
           valid, and matches the first empty line.)  The
           terminating string must appear by itself (unquoted and
           with no surrounding whitespace) on the terminating
           line.

           If the terminating string is quoted, the type of
           quotes used determine the treatment of the text.

           Double Quotes
               Double quotes indicate that the text will be
               interpolated using exactly the same rules as
               normal double quoted strings.




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                      print <<EOF;
                   The price is $Price.
                   EOF

                      print << "EOF"; # same as above
                   The price is $Price.
                   EOF

           Single Quotes
               Single quotes indicate the text is to be treated
               literally with no interpolation of its content.
               This is similar to single quoted strings except
               that backslashes have no special meaning, with
               "\\" being treated as two backslashes and not one
               as they would in every other quoting construct.

               This is the only form of quoting in perl where
               there is no need to worry about escaping content,
               something that code generators can and do make
               good use of.

           Backticks
               The content of the here doc is treated just as it
               would be if the string were embedded in backticks.
               Thus the content is interpolated as though it were
               double quoted and then executed via the shell,
               with the results of the execution returned.

                      print << `EOC`; # execute command and get results
                   echo hi there
                   EOC

           It is possible to stack multiple here-docs in a row:

                  print <<"foo", <<"bar"; # you can stack them
               I said foo.
               foo
               I said bar.
               bar

                  myfunc(<< "THIS", 23, <<'THAT');
               Here's a line
               or two.
               THIS
               and here's another.
               THAT

           Just don't forget that you have to put a semicolon on
           the end to finish the statement, as Perl doesn't know
           you're not going to try to do this:







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                  print <<ABC
               179231
               ABC
                  + 20;

           If you want to remove the line terminator from your
           here-docs, use "chomp()".

               chomp($string = <<'END');
               This is a string.
               END

           If you want your here-docs to be indented with the
           rest of the code, you'll need to remove leading
           whitespace from each line manually:

               ($quote = <<'FINIS') =~ s/^\s+//gm;
                  The Road goes ever on and on,
                  down from the door where it began.
               FINIS

           If you use a here-doc within a delimited construct,
           such as in "s///eg", the quoted material must come on
           the lines following the final delimiter.  So instead
           of

               s/this/<<E . 'that'
               the other
               E
                . 'more '/eg;

           you have to write

               s/this/<<E . 'that'
                . 'more '/eg;
               the other
               E

           If the terminating identifier is on the last line of
           the program, you must be sure there is a newline after
           it; otherwise, Perl will give the warning Can't find
           string terminator "END" anywhere before EOF....

           Additionally, the quoting rules for the end of string
           identifier are not related to Perl's quoting rules --
           "q()", "qq()", and the like are not supported in place
           of '' and "", and the only interpolation is for
           backslashing the quoting character:

               print << "abc\"def";
               testing...
               abc"def

           Finally, quoted strings cannot span multiple lines.



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           The general rule is that the identifier must be a
           string literal.  Stick with that, and you should be
           safe.

   Gory details of parsing quoted constructs
       When presented with something that might have several
       different interpretations, Perl uses the DWIM (that's "Do
       What I Mean") principle to pick the most probable
       interpretation.  This strategy is so successful that Perl
       programmers often do not suspect the ambivalence of what
       they write.  But from time to time, Perl's notions differ
       substantially from what the author honestly meant.

       This section hopes to clarify how Perl handles quoted
       constructs.  Although the most common reason to learn this
       is to unravel labyrinthine regular expressions, because
       the initial steps of parsing are the same for all quoting
       operators, they are all discussed together.

       The most important Perl parsing rule is the first one
       discussed below: when processing a quoted construct, Perl
       first finds the end of that construct, then interprets its
       contents.  If you understand this rule, you may skip the
       rest of this section on the first reading.  The other
       rules are likely to contradict the user's expectations
       much less frequently than this first one.

       Some passes discussed below are performed concurrently,
       but because their results are the same, we consider them
       individually.  For different quoting constructs, Perl
       performs different numbers of passes, from one to four,
       but these passes are always performed in the same order.

       Finding the end
           The first pass is finding the end of the quoted
           construct, where the information about the delimiters
           is used in parsing.  During this search, text between
           the starting and ending delimiters is copied to a safe
           location. The text copied gets delimiter-independent.

           If the construct is a here-doc, the ending delimiter
           is a line that has a terminating string as the
           content. Therefore "<<EOF" is terminated by "EOF"
           immediately followed by "\n" and starting from the
           first column of the terminating line.  When searching
           for the terminating line of a here-doc, nothing is
           skipped. In other words, lines after the here-doc
           syntax are compared with the terminating string line
           by line.

           For the constructs except here-docs, single characters
           are used as starting and ending delimiters. If the
           starting delimiter is an opening punctuation (that is
           "(", "[", "{", or "<"), the ending delimiter is the



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           corresponding closing punctuation (that is ")", "]",
           "}", or ">").  If the starting delimiter is an
           unpaired character like "/" or a closing punctuation,
           the ending delimiter is same as the starting
           delimiter.  Therefore a "/" terminates a "qq//"
           construct, while a "]" terminates "qq[]" and "qq]]"
           constructs.

           When searching for single-character delimiters,
           escaped delimiters and "\\" are skipped. For example,
           while searching for terminating "/", combinations of
           "\\" and "\/" are skipped.  If the delimiters are
           bracketing, nested pairs are also skipped.  For
           example, while searching for closing "]" paired with
           the opening "[", combinations of "\\", "\]", and "\["
           are all skipped, and nested "[" and "]" are skipped as
           well.  However, when backslashes are used as the
           delimiters (like "qq\\" and "tr\\\"), nothing is
           skipped.  During the search for the end, backslashes
           that escape delimiters are removed (exactly speaking,
           they are not copied to the safe location).

           For constructs with three-part delimiters ("s///",
           "y///", and "tr///"), the search is repeated once
           more.  If the first delimiter is not an opening
           punctuation, three delimiters must be same such as
           "s!!!" and "tr)))", in which case the second delimiter
           terminates the left part and starts the right part at
           once.  If the left part is delimited by bracketing
           punctuations (that is "()", "[]", "{}", or "<>"), the
           right part needs another pair of delimiters such as
           "s(){}" and "tr[]//".  In these cases, whitespaces and
           comments are allowed between both parts, though the
           comment must follow at least one whitespace; otherwise
           a character expected as the start of the comment may
           be regarded as the starting delimiter of the right
           part.

           During this search no attention is paid to the
           semantics of the construct.  Thus:

               "$hash{"$foo/$bar"}"

           or:

               m/
                 bar       # NOT a comment, this slash / terminated m//!
                /x

           do not form legal quoted expressions.   The quoted
           part ends on the first """ and "/", and the rest
           happens to be a syntax error.  Because the slash that
           terminated "m//" was followed by a "SPACE", the
           example above is not "m//x", but rather "m//" with no



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           "/x" modifier.  So the embedded "#" is interpreted as
           a literal "#".

           Also no attention is paid to "\c\" (multichar control
           char syntax) during this search. Thus the second "\"
           in "qq/\c\/" is interpreted as a part of "\/", and the
           following "/" is not recognized as a delimiter.
           Instead, use "\034" or "\x1c" at the end of quoted
           constructs.

       Interpolation
           The next step is interpolation in the text obtained,
           which is now delimiter-independent.  There are
           multiple cases.

           "<<'EOF'"
               No interpolation is performed.  Note that the
               combination "\\" is left intact, since escaped
               delimiters are not available for here-docs.

           "m''", the pattern of "s'''"
               No interpolation is performed at this stage.  Any
               backslashed sequences including "\\" are treated
               at the stage to "parsing regular expressions".

           '', "q//", "tr'''", "y'''", the replacement of "s'''"
               The only interpolation is removal of "\" from
               pairs of "\\".  Therefore "-" in "tr'''" and
               "y'''" is treated literally as a hyphen and no
               character range is available.  "\1" in the
               replacement of "s'''" does not work as $1.

           "tr///", "y///"
               No variable interpolation occurs.  String
               modifying combinations for case and quoting such
               as "\Q", "\U", and "\E" are not recognized.  The
               other escape sequences such as "\200" and "\t" and
               backslashed characters such as "\\" and "\-" are
               converted to appropriate literals.  The character
               "-" is treated specially and therefore "\-" is
               treated as a literal "-".

           "", "``", "qq//", "qx//", "<file*glob>", "<<"EOF""
               "\Q", "\U", "\u", "\L", "\l" (possibly paired with
               "\E") are converted to corresponding Perl
               constructs.  Thus, "$foo\Qbaz$bar" is converted to
               "$foo . (quotemeta("baz" . $bar))" internally.
               The other escape sequences such as "\200" and "\t"
               and backslashed characters such as "\\" and "\-"
               are replaced with appropriate expansions.

               Let it be stressed that whatever falls between
               "\Q" and "\E" is interpolated in the usual way.
               Something like "\Q\\E" has no "\E" inside.



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               instead, it has "\Q", "\\", and "E", so the result
               is the same as for "\\\\E".  As a general rule,
               backslashes between "\Q" and "\E" may lead to
               counterintuitive results.  So, "\Q\t\E" is
               converted to "quotemeta("\t")", which is the same
               as "\\\t" (since TAB is not alphanumeric).  Note
               also that:

                 $str = '\t';
                 return "\Q$str";

               may be closer to the conjectural intention of the
               writer of "\Q\t\E".

               Interpolated scalars and arrays are converted
               internally to the "join" and "." catenation
               operations.  Thus, "$foo XXX '@arr'" becomes:

                 $foo . " XXX '" . (join $", @arr) . "'";

               All operations above are performed simultaneously,
               left to right.

               Because the result of "\Q STRING \E" has all
               metacharacters quoted, there is no way to insert a
               literal "$" or "@" inside a "\Q\E" pair.  If
               protected by "\", "$" will be quoted to became
               "\\\$"; if not, it is interpreted as the start of
               an interpolated scalar.

               Note also that the interpolation code needs to
               make a decision on where the interpolated scalar
               ends.  For instance, whether "a $b -> {c}" really
               means:

                 "a " . $b . " -> {c}";

               or:

                 "a " . $b -> {c};

               Most of the time, the longest possible text that
               does not include spaces between components and
               which contains matching braces or brackets.
               because the outcome may be determined by voting
               based on heuristic estimators, the result is not
               strictly predictable.  Fortunately, it's usually
               correct for ambiguous cases.

           the replacement of "s///"
               Processing of "\Q", "\U", "\u", "\L", "\l", and
               interpolation happens as with "qq//" constructs.

               It is at this step that "\1" is begrudgingly



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               converted to $1 in the replacement text of "s///",
               in order to correct the incorrigible sed hackers
               who haven't picked up the saner idiom yet.  A
               warning is emitted if the "use warnings" pragma or
               the -w command-line flag (that is, the $^W
               variable) was set.

           "RE" in "?RE?", "/RE/", "m/RE/", "s/RE/foo/",
               Processing of "\Q", "\U", "\u", "\L", "\l", "\E",
               and interpolation happens (almost) as with "qq//"
               constructs.

               However any other combinations of "\" followed by
               a character are not substituted but only skipped,
               in order to parse them as regular expressions at
               the following step.  As "\c" is skipped at this
               step, "@" of "\c@" in RE is possibly treated as an
               array symbol (for example @foo), even though the
               same text in "qq//" gives interpolation of "\c@".

               Moreover, inside "(?{BLOCK})", "(?# comment )",
               and a "#"-comment in a "//x"-regular expression,
               no processing is performed whatsoever.  This is
               the first step at which the presence of the "//x"
               modifier is relevant.

               Interpolation in patterns has several quirks: $|,
               $(, $), "@+" and "@-" are not interpolated, and
               constructs $var[SOMETHING] are voted (by several
               different estimators) to be either an array
               element or $var followed by an RE alternative.
               This is where the notation "${arr[$bar]}" comes
               handy: "/${arr[0-9]}/" is interpreted as array
               element "-9", not as a regular expression from the
               variable $arr followed by a digit, which would be
               the interpretation of "/$arr[0-9]/".  Since voting
               among different estimators may occur, the result
               is not predictable.

               The lack of processing of "\\" creates specific
               restrictions on the post-processed text.  If the
               delimiter is "/", one cannot get the combination
               "\/" into the result of this step.  "/" will
               finish the regular expression, "\/" will be
               stripped to "/" on the previous step, and "\\/"
               will be left as is.  Because "/" is equivalent to
               "\/" inside a regular expression, this does not
               matter unless the delimiter happens to be
               character special to the RE engine, such as in
               "s*foo*bar*", "m[foo]", or "?foo?"; or an
               alphanumeric char, as in:

                 m m ^ a \s* b mmx;




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               In the RE above, which is intentionally obfuscated
               for illustration, the delimiter is "m", the
               modifier is "mx", and after delimiter-removal the
               RE is the same as for "m/ ^ a \s* b /mx".  There's
               more than one reason you're encouraged to restrict
               your delimiters to non-alphanumeric, non-
               whitespace choices.

           This step is the last one for all constructs except
           regular expressions, which are processed further.

       parsing regular expressions
           Previous steps were performed during the compilation
           of Perl code, but this one happens at run
           time--although it may be optimized to be calculated at
           compile time if appropriate.  After preprocessing
           described above, and possibly after evaluation if
           concatenation, joining, casing translation, or
           metaquoting are involved, the resulting string is
           passed to the RE engine for compilation.

           Whatever happens in the RE engine might be better
           discussed in perlre, but for the sake of continuity,
           we shall do so here.

           This is another step where the presence of the "//x"
           modifier is relevant.  The RE engine scans the string
           from left to right and converts it to a finite
           automaton.

           Backslashed characters are either replaced with
           corresponding literal strings (as with "\{"), or else
           they generate special nodes in the finite automaton
           (as with "\b").  Characters special to the RE engine
           (such as "|") generate corresponding nodes or groups
           of nodes.  "(?#...)" comments are ignored.  All the
           rest is either converted to literal strings to match,
           or else is ignored (as is whitespace and "#"-style
           comments if "//x" is present).

           Parsing of the bracketed character class construct,
           "[...]", is rather different than the rule used for
           the rest of the pattern.  The terminator of this
           construct is found using the same rules as for finding
           the terminator of a "{}"-delimited construct, the only
           exception being that "]" immediately following "[" is
           treated as though preceded by a backslash.  Similarly,
           the terminator of "(?{...})" is found using the same
           rules as for finding the terminator of a
           "{}"-delimited construct.

           It is possible to inspect both the string given to RE
           engine and the resulting finite automaton.  See the
           arguments "debug"/"debugcolor" in the "use re" pragma,



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           as well as Perl's -Dr command-line switch documented
           in "Command Switches" in perlrun.

       Optimization of regular expressions
           This step is listed for completeness only.  Since it
           does not change semantics, details of this step are
           not documented and are subject to change without
           notice.  This step is performed over the finite
           automaton that was generated during the previous pass.

           It is at this stage that "split()" silently optimizes
           "/^/" to mean "/^/m".

   I/O Operators
       There are several I/O operators you should know about.

       A string enclosed by backticks (grave accents) first
       undergoes double-quote interpolation.  It is then
       interpreted as an external command, and the output of that
       command is the value of the backtick string, like in a
       shell.  In scalar context, a single string consisting of
       all output is returned.  In list context, a list of values
       is returned, one per line of output.  (You can set $/ to
       use a different line terminator.)  The command is executed
       each time the pseudo-literal is evaluated.  The status
       value of the command is returned in $? (see perlvar for
       the interpretation of $?).  Unlike in csh, no translation
       is done on the return data--newlines remain newlines.
       Unlike in any of the shells, single quotes do not hide
       variable names in the command from interpretation.  To
       pass a literal dollar-sign through to the shell you need
       to hide it with a backslash.  The generalized form of
       backticks is "qx//".  (Because backticks always undergo
       shell expansion as well, see perlsec for security
       concerns.)

       In scalar context, evaluating a filehandle in angle
       brackets yields the next line from that file (the newline,
       if any, included), or "undef" at end-of-file or on error.
       When $/ is set to "undef" (sometimes known as file-slurp
       mode) and the file is empty, it returns '' the first time,
       followed by "undef" subsequently.

       Ordinarily you must assign the returned value to a
       variable, but there is one situation where an automatic
       assignment happens.  If and only if the input symbol is
       the only thing inside the conditional of a "while"
       statement (even if disguised as a "for(;;)" loop), the
       value is automatically assigned to the global variable $_,
       destroying whatever was there previously.  (This may seem
       like an odd thing to you, but you'll use the construct in
       almost every Perl script you write.)  The $_ variable is
       not implicitly localized.  You'll have to put a "local
       $_;" before the loop if you want that to happen.



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       The following lines are equivalent:

           while (defined($_ = <STDIN>)) { print; }
           while ($_ = <STDIN>) { print; }
           while (<STDIN>) { print; }
           for (;<STDIN>;) { print; }
           print while defined($_ = <STDIN>);
           print while ($_ = <STDIN>);
           print while <STDIN>;

       This also behaves similarly, but avoids $_ :

           while (my $line = <STDIN>) { print $line }

       In these loop constructs, the assigned value (whether
       assignment is automatic or explicit) is then tested to see
       whether it is defined.  The defined test avoids problems
       where line has a string value that would be treated as
       false by Perl, for example a "" or a "0" with no trailing
       newline.  If you really mean for such values to terminate
       the loop, they should be tested for explicitly:

           while (($_ = <STDIN>) ne '0') { ... }
           while (<STDIN>) { last unless $_; ... }

       In other boolean contexts, "<filehandle>" without an
       explicit "defined" test or comparison elicit a warning if
       the "use warnings" pragma or the -w command-line switch
       (the $^W variable) is in effect.

       The filehandles STDIN, STDOUT, and STDERR are predefined.
       (The filehandles "stdin", "stdout", and "stderr" will also
       work except in packages, where they would be interpreted
       as local identifiers rather than global.)  Additional
       filehandles may be created with the open() function,
       amongst others.  See perlopentut and "open" in perlfunc
       for details on this.

       If a <FILEHANDLE> is used in a context that is looking for
       a list, a list comprising all input lines is returned, one
       line per list element.  It's easy to grow to a rather
       large data space this way, so use with care.

       <FILEHANDLE> may also be spelled "readline(*FILEHANDLE)".
       See "readline" in perlfunc.

       The null filehandle <> is special: it can be used to
       emulate the behavior of sed and awk.  Input from <> comes
       either from standard input, or from each file listed on
       the command line.  Here's how it works: the first time <>
       is evaluated, the @ARGV array is checked, and if it is
       empty, $ARGV[0] is set to "-", which when opened gives you
       standard input.  The @ARGV array is then processed as a
       list of filenames.  The loop



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           while (<>) {
               ...                     # code for each line
           }

       is equivalent to the following Perl-like pseudo code:

           unshift(@ARGV, '-') unless @ARGV;
           while ($ARGV = shift) {
               open(ARGV, $ARGV);
               while (<ARGV>) {
                   ...         # code for each line
               }
           }

       except that it isn't so cumbersome to say, and will
       actually work.  It really does shift the @ARGV array and
       put the current filename into the $ARGV variable.  It also
       uses filehandle ARGV internally--<> is just a synonym for
       <ARGV>, which is magical.  (The pseudo code above doesn't
       work because it treats <ARGV> as non-magical.)

       Since the null filehandle uses the two argument form of
       "open" in perlfunc it interprets special characters, so if
       you have a script like this:

           while (<>) {
               print;
           }

       and call it with "perl dangerous.pl 'rm -rfv *|'", it
       actually opens a pipe, executes the "rm" command and reads
       "rm"'s output from that pipe.  If you want all items in
       @ARGV to be interpreted as file names, you can use the
       module "ARGV::readonly" from CPAN.

       You can modify @ARGV before the first <> as long as the
       array ends up containing the list of filenames you really
       want.  Line numbers ($.)  continue as though the input
       were one big happy file.  See the example in "eof" in
       perlfunc for how to reset line numbers on each file.

       If you want to set @ARGV to your own list of files, go
       right ahead.  This sets @ARGV to all plain text files if
       no @ARGV was given:

           @ARGV = grep { -f && -T } glob('*') unless @ARGV;

       You can even set them to pipe commands.  For example, this
       automatically filters compressed arguments through gzip:

           @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;

       If you want to pass switches into your script, you can use
       one of the Getopts modules or put a loop on the front like



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       this:

           while ($_ = $ARGV[0], /^-/) {
               shift;
               last if /^--$/;
               if (/^-D(.*)/) { $debug = $1 }
               if (/^-v/)     { $verbose++  }
               # ...           # other switches
           }

           while (<>) {
               # ...           # code for each line
           }

       The <> symbol will return "undef" for end-of-file only
       once.  If you call it again after this, it will assume you
       are processing another @ARGV list, and if you haven't set
       @ARGV, will read input from STDIN.

       If what the angle brackets contain is a simple scalar
       variable (e.g., <$foo>), then that variable contains the
       name of the filehandle to input from, or its typeglob, or
       a reference to the same.  For example:

           $fh = \*STDIN;
           $line = <$fh>;

       If what's within the angle brackets is neither a
       filehandle nor a simple scalar variable containing a
       filehandle name, typeglob, or typeglob reference, it is
       interpreted as a filename pattern to be globbed, and
       either a list of filenames or the next filename in the
       list is returned, depending on context.  This distinction
       is determined on syntactic grounds alone.  That means
       "<$x>" is always a readline() from an indirect handle, but
       "<$hash{key}>" is always a glob().  That's because $x is a
       simple scalar variable, but $hash{key} is not--it's a hash
       element.  Even "<$x >" (note the extra space) is treated
       as "glob("$x ")", not "readline($x)".

       One level of double-quote interpretation is done first,
       but you can't say "<$foo>" because that's an indirect
       filehandle as explained in the previous paragraph.  (In
       older versions of Perl, programmers would insert curly
       brackets to force interpretation as a filename glob:
       "<${foo}>".  These days, it's considered cleaner to call
       the internal function directly as "glob($foo)", which is
       probably the right way to have done it in the first
       place.)  For example:

           while (<*.c>) {
               chmod 0644, $_;
           }




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       is roughly equivalent to:

           open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
           while (<FOO>) {
               chomp;
               chmod 0644, $_;
           }

       except that the globbing is actually done internally using
       the standard "File::Glob" extension.  Of course, the
       shortest way to do the above is:

           chmod 0644, <*.c>;

       A (file)glob evaluates its (embedded) argument only when
       it is starting a new list.  All values must be read before
       it will start over.  In list context, this isn't important
       because you automatically get them all anyway.  However,
       in scalar context the operator returns the next value each
       time it's called, or "undef" when the list has run out.
       As with filehandle reads, an automatic "defined" is
       generated when the glob occurs in the test part of a
       "while", because legal glob returns (e.g. a file called 0)
       would otherwise terminate the loop.  Again, "undef" is
       returned only once.  So if you're expecting a single value
       from a glob, it is much better to say

           ($file) = <blurch*>;

       than

           $file = <blurch*>;

       because the latter will alternate between returning a
       filename and returning false.

       If you're trying to do variable interpolation, it's
       definitely better to use the glob() function, because the
       older notation can cause people to become confused with
       the indirect filehandle notation.

           @files = glob("$dir/*.[ch]");
           @files = glob($files[$i]);

   Constant Folding
       Like C, Perl does a certain amount of expression
       evaluation at compile time whenever it determines that all
       arguments to an operator are static and have no side
       effects.  In particular, string concatenation happens at
       compile time between literals that don't do variable
       substitution.  Backslash interpolation also happens at
       compile time.  You can say





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           'Now is the time for all' . "\n" .
               'good men to come to.'

       and this all reduces to one string internally.  Likewise,
       if you say

           foreach $file (@filenames) {
               if (-s $file > 5 + 100 * 2**16) {  }
           }

       the compiler will precompute the number which that
       expression represents so that the interpreter won't have
       to.

   No-ops
       Perl doesn't officially have a no-op operator, but the
       bare constants 0 and 1 are special-cased to not produce a
       warning in a void context, so you can for example safely
       do

           1 while foo();

   Bitwise String Operators
       Bitstrings of any size may be manipulated by the bitwise
       operators ("~ | & ^").

       If the operands to a binary bitwise op are strings of
       different sizes, | and ^ ops act as though the shorter
       operand had additional zero bits on the right, while the &
       op acts as though the longer operand were truncated to the
       length of the shorter.  The granularity for such extension
       or truncation is one or more bytes.

           # ASCII-based examples
           print "j p \n" ^ " a h";            # prints "JAPH\n"
           print "JA" | "  ph\n";              # prints "japh\n"
           print "japh\nJunk" & '_____';       # prints "JAPH\n";
           print 'p N$' ^ " E<H\n";            # prints "Perl\n";

       If you are intending to manipulate bitstrings, be certain
       that you're supplying bitstrings: If an operand is a
       number, that will imply a numeric bitwise operation.  You
       may explicitly show which type of operation you intend by
       using "" or "0+", as in the examples below.

           $foo =  150  |  105;        # yields 255  (0x96 | 0x69 is 0xFF)
           $foo = '150' |  105;        # yields 255
           $foo =  150  | '105';       # yields 255
           $foo = '150' | '105';       # yields string '155' (under ASCII)

           $baz = 0+$foo & 0+$bar;     # both ops explicitly numeric
           $biz = "$foo" ^ "$bar";     # both ops explicitly stringy

       See "vec" in perlfunc for information on how to manipulate



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       individual bits in a bit vector.

   Integer Arithmetic
       By default, Perl assumes that it must do most of its
       arithmetic in floating point.  But by saying

           use integer;

       you may tell the compiler that it's okay to use integer
       operations (if it feels like it) from here to the end of
       the enclosing BLOCK.  An inner BLOCK may countermand this
       by saying

           no integer;

       which lasts until the end of that BLOCK.  Note that this
       doesn't mean everything is only an integer, merely that
       Perl may use integer operations if it is so inclined.  For
       example, even under "use integer", if you take the
       sqrt(2), you'll still get 1.4142135623731 or so.

       Used on numbers, the bitwise operators ("&", "|", "^",
       "~", "<<", and ">>") always produce integral results.
       (But see also "Bitwise String Operators".)  However, "use
       integer" still has meaning for them.  By default, their
       results are interpreted as unsigned integers, but if "use
       integer" is in effect, their results are interpreted as
       signed integers.  For example, "~0" usually evaluates to a
       large integral value.  However, "use integer; ~0" is "-1"
       on two's-complement machines.

   Floating-point Arithmetic
       While "use integer" provides integer-only arithmetic,
       there is no analogous mechanism to provide automatic
       rounding or truncation to a certain number of decimal
       places.  For rounding to a certain number of digits,
       sprintf() or printf() is usually the easiest route.  See
       perlfaq4.

       Floating-point numbers are only approximations to what a
       mathematician would call real numbers.  There are
       infinitely more reals than floats, so some corners must be
       cut.  For example:

           printf "%.20g\n", 123456789123456789;
           #        produces 123456789123456784

       Testing for exact equality of floating-point equality or
       inequality is not a good idea.  Here's a (relatively
       expensive) work-around to compare whether two floating-
       point numbers are equal to a particular number of decimal
       places.  See Knuth, volume II, for a more robust treatment
       of this topic.




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           sub fp_equal {
               my ($X, $Y, $POINTS) = @_;
               my ($tX, $tY);
               $tX = sprintf("%.${POINTS}g", $X);
               $tY = sprintf("%.${POINTS}g", $Y);
               return $tX eq $tY;
           }

       The POSIX module (part of the standard perl distribution)
       implements ceil(), floor(), and other mathematical and
       trigonometric functions.  The Math::Complex module (part
       of the standard perl distribution) defines mathematical
       functions that work on both the reals and the imaginary
       numbers.  Math::Complex not as efficient as POSIX, but
       POSIX can't work with complex numbers.

       Rounding in financial applications can have serious
       implications, and the rounding method used should be
       specified precisely.  In these cases, it probably pays not
       to trust whichever system rounding is being used by Perl,
       but to instead implement the rounding function you need
       yourself.

   Bigger Numbers
       The standard Math::BigInt and Math::BigFloat modules
       provide variable-precision arithmetic and overloaded
       operators, although they're currently pretty slow. At the
       cost of some space and considerable speed, they avoid the
       normal pitfalls associated with limited-precision
       representations.

           use Math::BigInt;
           $x = Math::BigInt->new('123456789123456789');
           print $x * $x;

           # prints +15241578780673678515622620750190521

       There are several modules that let you calculate with
       (bound only by memory and cpu-time) unlimited or fixed
       precision. There are also some non-standard modules that
       provide faster implementations via external C libraries.

       Here is a short, but incomplete summary:














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               Math::Fraction          big, unlimited fractions like 9973 / 12967
               Math::String            treat string sequences like numbers
               Math::FixedPrecision    calculate with a fixed precision
               Math::Currency          for currency calculations
               Bit::Vector             manipulate bit vectors fast (uses C)
               Math::BigIntFast        Bit::Vector wrapper for big numbers
               Math::Pari              provides access to the Pari C library
               Math::BigInteger        uses an external C library
               Math::Cephes            uses external Cephes C library (no big numbers)
               Math::Cephes::Fraction  fractions via the Cephes library
               Math::GMP               another one using an external C library

       Choose wisely.












































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