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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 pro-
       vides 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, ele-
       gant, 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 deter-
       mine that Perl is different things to different people, but that lots of people think it's at least worth writ-
       ing 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

       ... 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).

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

       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:

           # 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.

           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.

       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.

           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 prac-
       tice.  "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 $a = "foo";
           if ($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

       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 $b" would
       cause a compile-time error and prevent you from running the program.  Using "strict" is highly recommended.

       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":

               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";
               }

       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
               ==  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 sup-
           ported as operators in their own right.  They're more readable than the C-style operators, but have differ-
           ent 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(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: $!";

       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  = <INFILE>;
           my @lines = <INFILE>;

       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 (<INFILE>) {     # 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 OUTFILE $record;
           print LOGFILE $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 INFILE;

       Regular expressions

       Perl's regular expression support is both broad and deep, and is the subject of lengthy documentation in perl-
       requick, 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.

       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 metacharac-
           ters 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.

       Writing subroutines

       Writing subroutines is easy:

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

       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;
           }

       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 Mod-
       ule::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 installa-
       tion.

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

AUTHOR
       Kirrily "Skud" Robert <skudATcpan.org>



perl v5.8.8                       2006-01-07                      PERLINTRO(1)