perl progname.pl

   #!/usr/bin/env perl

    print "Hello, world";

    # This is a comment

    print 
        "Hello, world"
        ;

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

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

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

    print 42;

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

• 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.

  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";

  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;

  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.

    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";

    my $var = "value";

    $var = "value";

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

• 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 :

      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";
      }

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

• Arithmetic

      +   addition
      -   subtraction
      *   multiplication
      /   division

• Numeric comparison

      ==  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)

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

    open(INFILE,  "input.txt")   or die "Can't open input.txt: $!";
    open(OUTFILE, ">output.txt") or die "Can't open output.txt: $!";
    open(LOGFILE, ">>my.log")    or die "Can't open logfile: $!";

    my $line  = <INFILE>
    my @lines = <INFILE>

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

    print STDERR "This is your final warning.\n";
    print OUTFILE $record;
    print LOGFILE $logmessage;

    close INFILE;

• 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.

• 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.

      # 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.

    sub log {
        my $logmessage = shift;
        print LOGFILE $logmessage;
    }

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

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

For more information on writing subroutines, see perlsub.

OO Perl

---

OO Perl is relatively simple and is implemented using 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>