


PERLINTRO(1)     Perl Programmers Reference Guide    PERLINTRO(1)


NAME
       perlintro -- a brief introduction and overview of Perl

DESCRIPTION
       This document is intended to give you a quick overview of
       the Perl programming language, along with pointers to
       further documentation.  It is intended as a "bootstrap"
       guide for those who are new to the language, and provides
       just enough information for you to be able to read other
       peoples' Perl and understand roughly what it's doing, or
       write your own simple scripts.

       This introductory document does not aim to be complete.
       It does not even aim to be entirely accurate.  In some
       cases perfection has been sacrificed in the goal of
       getting the general idea across.  You are strongly advised
       to follow this introduction with more information from the
       full Perl manual, the table of contents to which can be
       found in perltoc.

       Throughout this document you'll see references to other
       parts of the Perl documentation.  You can read that
       documentation using the "perldoc" command or whatever
       method you're using to read this document.

   What is Perl?
       Perl is a general-purpose programming language originally
       developed for text manipulation and now used for a wide
       range of tasks including system administration, web
       development, network programming, GUI development, and
       more.

       The language is intended to be practical (easy to use,
       efficient, complete) rather than beautiful (tiny, elegant,
       minimal).  Its major features are that it's easy to use,
       supports both procedural and object-oriented (OO)
       programming, has powerful built-in support for text
       processing, and has one of the world's most impressive
       collections of third-party modules.

       Different definitions of Perl are given in perl, perlfaq1
       and no doubt other places.  From this we can determine
       that Perl is different things to different people, but
       that lots of people think it's at least worth writing
       about.

   Running Perl programs
       To run a Perl program from the Unix command line:

           perl progname.pl

       Alternatively, put this as the first line of your script:

           #!/usr/bin/env perl



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       ... and run the script as "/path/to/script.pl".  Of
       course, it'll need to be executable first, so "chmod 755
       script.pl" (under Unix).

       (This start line assumes you have the env program. You can
       also put directly the path to your perl executable, like
       in "#!/usr/bin/perl").

       For more information, including instructions for other
       platforms such as Windows and Mac OS, read perlrun.

   Safety net
       Perl by default is very forgiving. In order to make it
       more robust it is recommended to start every program with
       the following lines:

           #!/usr/bin/perl
           use strict;
           use warnings;

       The two additional lines request from perl to catch
       various common problems in your code. They check different
       things so you need both. A potential problem caught by
       "use strict;" will cause your code to stop immediately
       when it is encountered, while "use warnings;" will merely
       give a warning (like the command-line switch -w) and let
       your code run.  To read more about them check their
       respective manual pages at strict and warnings.

   Basic syntax overview
       A Perl script or program consists of one or more
       statements.  These statements are simply written in the
       script in a straightforward fashion.  There is no need to
       have a "main()" function or anything of that kind.

       Perl statements end in a semi-colon:

           print "Hello, world";

       Comments start with a hash symbol and run to the end of
       the line

           # This is a comment

       Whitespace is irrelevant:

           print
               "Hello, world"
               ;

       ... except inside quoted strings:






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           # this would print with a linebreak in the middle
           print "Hello
           world";

       Double quotes or single quotes may be used around literal
       strings:

           print "Hello, world";
           print 'Hello, world';

       However, only double quotes "interpolate" variables and
       special characters such as newlines ("\n"):

           print "Hello, $name\n";     # works fine
           print 'Hello, $name\n';     # prints $name\n literally

       Numbers don't need quotes around them:

           print 42;

       You can use parentheses for functions' arguments or omit
       them according to your personal taste.  They are only
       required occasionally to clarify issues of precedence.

           print("Hello, world\n");
           print "Hello, world\n";

       More detailed information about Perl syntax can be found
       in perlsyn.

   Perl variable types
       Perl has three main variable types: scalars, arrays, and
       hashes.

       Scalars
           A scalar represents a single value:

               my $animal = "camel";
               my $answer = 42;

           Scalar values can be strings, integers or floating
           point numbers, and Perl will automatically convert
           between them as required.  There is no need to pre-
           declare your variable types, but you have to declare
           them using the "my" keyword the first time you use
           them. (This is one of the requirements of "use
           strict;".)

           Scalar values can be used in various ways:

               print $animal;
               print "The animal is $animal\n";
               print "The square of $answer is ", $answer * $answer, "\n";




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           There are a number of "magic" scalars with names that
           look like punctuation or line noise.  These special
           variables are used for all kinds of purposes, and are
           documented in perlvar.  The only one you need to know
           about for now is $_ which is the "default variable".
           It's used as the default argument to a number of
           functions in Perl, and it's set implicitly by certain
           looping constructs.

               print;          # prints contents of $_ by default

       Arrays
           An array represents a list of values:

               my @animals = ("camel", "llama", "owl");
               my @numbers = (23, 42, 69);
               my @mixed   = ("camel", 42, 1.23);

           Arrays are zero-indexed.  Here's how you get at
           elements in an array:

               print $animals[0];              # prints "camel"
               print $animals[1];              # prints "llama"

           The special variable $#array tells you the index of
           the last element of an array:

               print $mixed[$#mixed];       # last element, prints 1.23

           You might be tempted to use "$#array + 1" to tell you
           how many items there are in an array.  Don't bother.
           As it happens, using @array where Perl expects to find
           a scalar value ("in scalar context") will give you the
           number of elements in the array:

               if (@animals < 5) { ... }

           The elements we're getting from the array start with a
           "$" because we're getting just a single value out of
           the array -- you ask for a scalar, you get a scalar.

           To get multiple values from an array:

               @animals[0,1];                  # gives ("camel", "llama");
               @animals[0..2];                 # gives ("camel", "llama", "owl");
               @animals[1..$#animals];         # gives all except the first element

           This is called an "array slice".

           You can do various useful things to lists:

               my @sorted    = sort @animals;
               my @backwards = reverse @numbers;




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           There are a couple of special arrays too, such as
           @ARGV (the command line arguments to your script) and
           @_ (the arguments passed to a subroutine).  These are
           documented in perlvar.

       Hashes
           A hash represents a set of key/value pairs:

               my %fruit_color = ("apple", "red", "banana", "yellow");

           You can use whitespace and the "=>" operator to lay
           them out more nicely:

               my %fruit_color = (
                   apple  => "red",
                   banana => "yellow",
               );

           To get at hash elements:

               $fruit_color{"apple"};           # gives "red"

           You can get at lists of keys and values with "keys()"
           and "values()".

               my @fruits = keys %fruit_colors;
               my @colors = values %fruit_colors;

           Hashes have no particular internal order, though you
           can sort the keys and loop through them.

           Just like special scalars and arrays, there are also
           special hashes.  The most well known of these is %ENV
           which contains environment variables.  Read all about
           it (and other special variables) in perlvar.

       Scalars, arrays and hashes are documented more fully in
       perldata.

       More complex data types can be constructed using
       references, which allow you to build lists and hashes
       within lists and hashes.

       A reference is a scalar value and can refer to any other
       Perl data type. So by storing a reference as the value of
       an array or hash element, you can easily create lists and
       hashes within lists and hashes. The following example
       shows a 2 level hash of hash structure using anonymous
       hash references.








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           my $variables = {
               scalar  =>  {
                            description => "single item",
                            sigil => '$',
                           },
               array   =>  {
                            description => "ordered list of items",
                            sigil => '@',
                           },
               hash    =>  {
                            description => "key/value pairs",
                            sigil => '%',
                           },
           };

           print "Scalars begin with a $variables->{'scalar'}->{'sigil'}\n";

       Exhaustive information on the topic of references can be
       found in perlreftut, perllol, perlref and perldsc.

   Variable scoping
       Throughout the previous section all the examples have used
       the syntax:

           my $var = "value";

       The "my" is actually not required; you could just use:

           $var = "value";

       However, the above usage will create global variables
       throughout your program, which is bad programming
       practice.  "my" creates lexically scoped variables
       instead.  The variables are scoped to the block (i.e. a
       bunch of statements surrounded by curly-braces) in which
       they are defined.

           my $x = "foo";
           my $some_condition = 1;
           if ($some_condition) {
               my $y = "bar";
               print $x;           # prints "foo"
               print $y;           # prints "bar"
           }
           print $x;               # prints "foo"
           print $y;               # prints nothing; $y has fallen out of scope

       Using "my" in combination with a "use strict;" at the top
       of your Perl scripts means that the interpreter will pick
       up certain common programming errors.  For instance, in
       the example above, the final "print $y" would cause a
       compile-time error and prevent you from running the
       program.  Using "strict" is highly recommended.




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   Conditional and looping constructs
       Perl has most of the usual conditional and looping
       constructs except for case/switch (but if you really want
       it, there is a Switch module in Perl 5.8 and newer, and on
       CPAN. See the section on modules, below, for more
       information about modules and CPAN).

       The conditions can be any Perl expression.  See the list
       of operators in the next section for information on
       comparison and boolean logic operators, which are commonly
       used in conditional statements.

       if
               if ( condition ) {
                   ...
               } elsif ( other condition ) {
                   ...
               } else {
                   ...
               }

           There's also a negated version of it:

               unless ( condition ) {
                   ...
               }

           This is provided as a more readable version of "if
           (!condition)".

           Note that the braces are required in Perl, even if
           you've only got one line in the block.  However, there
           is a clever way of making your one-line conditional
           blocks more English like:

               # the traditional way
               if ($zippy) {
                   print "Yow!";
               }

               # the Perlish post-condition way
               print "Yow!" if $zippy;
               print "We have no bananas" unless $bananas;

       while
               while ( condition ) {
                   ...
               }

           There's also a negated version, for the same reason we
           have "unless":






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               until ( condition ) {
                   ...
               }

           You can also use "while" in a post-condition:

               print "LA LA LA\n" while 1;          # loops forever

       for Exactly like C:

               for ($i = 0; $i <= $max; $i++) {
                   ...
               }

           The C style for loop is rarely needed in Perl since
           Perl provides the more friendly list scanning
           "foreach" loop.

       foreach
               foreach (@array) {
                   print "This element is $_\n";
               }

               print $list[$_] foreach 0 .. $max;

               # you don't have to use the default $_ either...
               foreach my $key (keys %hash) {
                   print "The value of $key is $hash{$key}\n";
               }

       For more detail on looping constructs (and some that
       weren't mentioned in this overview) see perlsyn.

   Builtin operators and functions
       Perl comes with a wide selection of builtin functions.
       Some of the ones we've already seen include "print",
       "sort" and "reverse".  A list of them is given at the
       start of perlfunc and you can easily read about any given
       function by using "perldoc -f functionname".

       Perl operators are documented in full in perlop, but here
       are a few of the most common ones:

       Arithmetic
               +   addition
               -   subtraction
               *   multiplication
               /   division

       Numeric comparison







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               ==  equality
               !=  inequality
               <   less than
               >   greater than
               <=  less than or equal
               >=  greater than or equal

       String comparison
               eq  equality
               ne  inequality
               lt  less than
               gt  greater than
               le  less than or equal
               ge  greater than or equal

           (Why do we have separate numeric and string
           comparisons?  Because we don't have special variable
           types, and Perl needs to know whether to sort
           numerically (where 99 is less than 100) or
           alphabetically (where 100 comes before 99).

       Boolean logic
               &&  and
               ||  or
               !   not

           ("and", "or" and "not" aren't just in the above table
           as descriptions of the operators -- they're also
           supported as operators in their own right.  They're
           more readable than the C-style operators, but have
           different precedence to "&&" and friends.  Check
           perlop for more detail.)

       Miscellaneous
               =   assignment
               .   string concatenation
               x   string multiplication
               ..  range operator (creates a list of numbers)

       Many operators can be combined with a "=" as follows:

           $a += 1;        # same as $a = $a + 1
           $a -= 1;        # same as $a = $a - 1
           $a .= "\n";     # same as $a = $a . "\n";

   Files and I/O
       You can open a file for input or output using the "open()"
       function.  It's documented in extravagant detail in
       perlfunc and perlopentut, but in short:

           open(my $in,  "<",  "input.txt")  or die "Can't open input.txt: $!";
           open(my $out, ">",  "output.txt") or die "Can't open output.txt: $!";
           open(my $log, ">>", "my.log")     or die "Can't open my.log: $!";




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       You can read from an open filehandle using the "<>"
       operator.  In scalar context it reads a single line from
       the filehandle, and in list context it reads the whole
       file in, assigning each line to an element of the list:

           my $line  = <$in>;
           my @lines = <$in>;

       Reading in the whole file at one time is called slurping.
       It can be useful but it may be a memory hog. Most text
       file processing can be done a line at a time with Perl's
       looping constructs.

       The "<>" operator is most often seen in a "while" loop:

           while (<$in>) {     # assigns each line in turn to $_
               print "Just read in this line: $_";
           }

       We've already seen how to print to standard output using
       "print()".  However, "print()" can also take an optional
       first argument specifying which filehandle to print to:

           print STDERR "This is your final warning.\n";
           print $out $record;
           print $log $logmessage;

       When you're done with your filehandles, you should
       "close()" them (though to be honest, Perl will clean up
       after you if you forget):

           close $in or die "$in: $!";

   Regular expressions
       Perl's regular expression support is both broad and deep,
       and is the subject of lengthy documentation in
       perlrequick, perlretut, and elsewhere.  However, in short:

       Simple matching
               if (/foo/)       { ... }  # true if $_ contains "foo"
               if ($a =~ /foo/) { ... }  # true if $a contains "foo"

           The "//" matching operator is documented in perlop.
           It operates on $_ by default, or can be bound to
           another variable using the "=~" binding operator (also
           documented in perlop).

       Simple substitution
               s/foo/bar/;               # replaces foo with bar in $_
               $a =~ s/foo/bar/;         # replaces foo with bar in $a
               $a =~ s/foo/bar/g;        # replaces ALL INSTANCES of foo with bar in $a

           The "s///" substitution operator is documented in
           perlop.



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       More complex regular expressions
           You don't just have to match on fixed strings.  In
           fact, you can match on just about anything you could
           dream of by using more complex regular expressions.
           These are documented at great length in perlre, but
           for the meantime, here's a quick cheat sheet:

               .                   a single character
               \s                  a whitespace character (space, tab, newline, ...)
               \S                  non-whitespace character
               \d                  a digit (0-9)
               \D                  a non-digit
               \w                  a word character (a-z, A-Z, 0-9, _)
               \W                  a non-word character
               [aeiou]             matches a single character in the given set
               [^aeiou]            matches a single character outside the given set
               (foo|bar|baz)       matches any of the alternatives specified

               ^                   start of string
               $                   end of string

           Quantifiers can be used to specify how many of the
           previous thing you want to match on, where "thing"
           means either a literal character, one of the
           metacharacters listed above, or a group of characters
           or metacharacters in parentheses.

               *                   zero or more of the previous thing
               +                   one or more of the previous thing
               ?                   zero or one of the previous thing
               {3}                 matches exactly 3 of the previous thing
               {3,6}               matches between 3 and 6 of the previous thing
               {3,}                matches 3 or more of the previous thing

           Some brief examples:

               /^\d+/              string starts with one or more digits
               /^$/                nothing in the string (start and end are adjacent)
               /(\d\s){3}/         a three digits, each followed by a whitespace
                                   character (eg "3 4 5 ")
               /(a.)+/             matches a string in which every odd-numbered letter
                                   is a (eg "abacadaf")

               # This loop reads from STDIN, and prints non-blank lines:
               while (<>) {
                   next if /^$/;
                   print;
               }

       Parentheses for capturing
           As well as grouping, parentheses serve a second
           purpose.  They can be used to capture the results of
           parts of the regexp match for later use.  The results
           end up in $1, $2 and so on.



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               # a cheap and nasty way to break an email address up into parts

               if ($email =~ /([^@]+)@(.+)/) {
                   print "Username is $1\n";
                   print "Hostname is $2\n";
               }

       Other regexp features
           Perl regexps also support backreferences, lookaheads,
           and all kinds of other complex details.  Read all
           about them in perlrequick, perlretut, and perlre.

   Writing subroutines
       Writing subroutines is easy:

           sub logger {
               my $logmessage = shift;
               open my $logfile, ">>", "my.log" or die "Could not open my.log: $!";
               print $logfile $logmessage;
           }

       Now we can use the subroutine just as any other built-in
       function:

           logger("We have a logger subroutine!");

       What's that "shift"?  Well, the arguments to a subroutine
       are available to us as a special array called @_ (see
       perlvar for more on that).  The default argument to the
       "shift" function just happens to be @_.  So "my
       $logmessage = shift;" shifts the first item off the list
       of arguments and assigns it to $logmessage.

       We can manipulate @_ in other ways too:

           my ($logmessage, $priority) = @_;       # common
           my $logmessage = $_[0];                 # uncommon, and ugly

       Subroutines can also return values:

           sub square {
               my $num = shift;
               my $result = $num * $num;
               return $result;
           }

       Then use it like:

           $sq = square(8);

       For more information on writing subroutines, see perlsub.

   OO Perl
       OO Perl is relatively simple and is implemented using



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       references which know what sort of object they are based
       on Perl's concept of packages.  However, OO Perl is
       largely beyond the scope of this document.  Read perlboot,
       perltoot, perltooc and perlobj.

       As a beginning Perl programmer, your most common use of OO
       Perl will be in using third-party modules, which are
       documented below.

   Using Perl modules
       Perl modules provide a range of features to help you avoid
       reinventing the wheel, and can be downloaded from CPAN (
       http://www.cpan.org/ ).  A number of popular modules are
       included with the Perl distribution itself.

       Categories of modules range from text manipulation to
       network protocols to database integration to graphics.  A
       categorized list of modules is also available from CPAN.

       To learn how to install modules you download from CPAN,
       read perlmodinstall.

       To learn how to use a particular module, use "perldoc
       Module::Name".  Typically you will want to "use
       Module::Name", which will then give you access to exported
       functions or an OO interface to the module.

       perlfaq contains questions and answers related to many
       common tasks, and often provides suggestions for good CPAN
       modules to use.

       perlmod describes Perl modules in general.  perlmodlib
       lists the modules which came with your Perl installation.

       If you feel the urge to write Perl modules, perlnewmod
       will give you good advice.

AUTHOR
       Kirrily "Skud" Robert <skud@cpan.org>


















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