Perl

Perl, also Practical Extraction and Report Language (a backronym, see below), is a programming language released by Larry Wall on December 18, 1987 that borrows features from C, sed, awk, shell scripting (sh), and (to a lesser extent) from many other programming languages.

Rationale

Perl was designed to be a practical language to extract information from text files and to generate reports from that information. One of its mottos is "There's more than one way to do it" (TMTOWTDI - pronounced 'Tim Toady'). Another is Perl: the Swiss Army Chainsaw of Programming Languages. One stated design goal is to make easy tasks easy and difficult tasks possible. Its versatility permits versions of many programming paradigms: procedural, functional, and object-oriented (though some claim that Perl is not a cleanly designed language because of its multiple paradigms). Perl has a powerful regular expression engine built directly into its syntax. Perl is often considered the archetypal scripting language and has been called the "glue that holds the web together", as it is one of the most popular CGI languages. Its function as a "glue language" can be described broadly as its ability to tie together different systems and interfaces that were not designed to interoperate.

Perl is one of the programming language components of the popular LAMP free software platform for web development.

Perl is free software, available under a combination of the Artistic License and the GPL. It is available for most operating systems but is particularly prevalent on Unix and Unix-like systems (e.g: Linux and FreeBSD), and is growing in popularity on Microsoft Windows systems. As an example of Perl in action, Wikipedia itself was a CGI script written in Perl until January 2002. Another example is Slashdot, which runs on the Perl-based Slashcode software. When used on the web, Perl is often used in conjunction with the Apache web server and its mod_perl module.

Perl is regarded by both its proponents and detractors as something of a grab bag of features and syntax. The difference between the two camps lies in whether this is seen as a virtue or a vice. Perl votaries maintain that this varied heritage is what makes the language so useful. Reference is often made to natural languages such as English and to evolution. For example, Larry Wall has argued that:

... we often joke that a camel is a horse designed by a committee, but if you think about it, the camel is pretty well adapted for life in the desert. The camel has evolved to be relatively self-sufficient. On the other hand, the camel has not evolved to smell good. Neither has Perl.

In recognition of its ugly-but-useful nature, Perl has adopted the camel as its mascot.

Implementation

A huge collection of freely usable perl modules, ranging from advanced mathematics to database connectivity, networking and more, can be downloaded from a network of sites called CPAN, an acronym for Comprehensive Perl Archive Network. Most or all of the software on CPAN is also available under either the Artistic License, the GPL, or both. CPAN.pm is also the name of the Perl module that downloads and installs other Perl modules from one of the CPAN mirror sites: such installations can be done with interactive prompts, or can be fully automated.

Although Perl has most of the ease-of-use features of an interpreted language, it does not strictly interpret and execute source code one line at a time. Rather, perl (the program) first compiles an entire program to an intermediate byte code (much like Java's byte code), optimizing as it goes, and then executes that byte code. It is possible to compile a Perl program to byte code to save the compilation step on later executions, though the "interpreter" is still needed to execute that code.

Current Version

The current version, 5.8.4, includes Unicode support. Development of the next major release, Perl 6, is also underway. It will run on Parrot, a virtual machine which is being developed as a possible multi-language target architecture.

Control structures

The basic control structures do not differ greatly from those used in the C or Java programming languages:

Loops

 while (Boolean expression) {
     statement(s)
 }

 do {
     statement(s)
 } while (Boolean expression);

 do {
     statement(s)
 } until (Boolean expression);

 for (initialisation ; termination condition ; incrementing expr) {
     statement(s)
 }

 foreach ( array ) {
     statement(s)
 }

If-then-statements

 if (Boolean expression) {
     statement(s)
 }

 unless (Boolean expression) {
     statement(s)
 }

 if (Boolean expression) {
     statement(s)
 } else {
     statement(s)
 }

 if (Boolean expression) {
     statement(s)
 } elsif (Boolean expression) {
     statement(s)
 }

For one-line statements, "until", "while" and "if" can also be used as follows:

 statement(s) until Boolean expression;
 statement(s) while Boolean expression;
 statement(s) if Boolean expression;

Subroutines

Subroutines in Perl can be specified with the keyword 'sub'. Variables passed to a subroutine appear in the subroutine as elements of the local (to the subroutine) scalar array @_. Calling a subroutine with three scalar variables results in array elements @_[0], @_[1], and @_[2] within the subroutine. Note that these elements would be referred to as the scalars $_[0], $_[1], and $_[2]. Also shift can be used, without specifying @_, to obtain each value.

Changes to elements in the @_ array within the subroutine are reflected in the elements in the calling program.

Subroutines naturally return the value of the last expression evaluated, though explicit use of the return statement is often encouraged for clarity.

An example subroutine definition and call follows:

 sub cube
 {
   my $x = shift;
   $x * $x * $x;
 }
 ...
 $z = -4;
 $y = cube($z);
 print "$y\n";

Named parameters can be simulated by passing a hash.

Perl and SQL databases

DBI/DBD modules can be used to access most ANSI SQL databases, including MySQL, PostgreSQL and Oracle.

Perl 5

Perl5, the most current production version of perl, is an interpreter which processes the text of a Perl script at runtime. Thus, the debugger is invoked directly from the command line with

        perl -dw ScriptName.pl Argument1 ... ...

Note that there is no limit to the number of arguments: Perl is polyadic; any number of arguments can be passed to any Perl subroutine, in general. This concept of "no arbitrary limits" is present in most other parts of the language as well. Perl can read a ten million byte string into a variable, if the machine has the memory for it.

Perl 6

Perl6 is currently under development, and is planned to separate parsing, compilation and runtime, making a virtual machine that is more attractive to developers looking to port other languages to the architecture.

Parrot is the Perl6 runtime, and can be programmed at a low level in Parrot assembly language. Parrot has existed in a limited form since December 2003. An increasing number of languages have been implemented to various degrees for the Parrot to be 'compiled' to Parrot assembly language opcodes. Besides a subset of the planned Perl6, these include BASIC, Befunge, Cola, Forth, Jako, Ook!, Plot, and even Python, Ruby, and Scheme.

Perl Code Samples

The canonical "hello world" program would be:

 #!/usr/bin/perl
 print "Hello world\n";

The first line is the shebang, which indicates the interpreter for Unix-like operating systems. The second line prints the string 'Hello world' and a newline (like a person pressing 'Return' or 'Enter').

Some people humorously claim that Perl stands for 'Pathologically Eclectic Rubbish Lister' due to the high use of meaningful punctuation characters in the language syntax, as may be seen in this example program to print a greeting:

 # A sample Perl program
 $_ = "Hello, world! The magic number is 234542354.\n";
 print;
 s/\d+/-1/;
 print;

and its output:

 Hello, world! The magic number is 234542354.
 Hello, world! The magic number is -1.

The fourth line of the example shows the use of a regular expression.

$string1 = "Hello World\n";
if ($string1 =~ m/...../) {
  print "$string1 has length >= 5\n";
}

$string1 = "Hello World\n";
if ($string1 =~ m/(H..).(o..)/) {
  print "We matched '$1' and '$2'\n";
}

Output:
We matched 'Hel' and 'o W';

$string1 = "Hello World\n";
if ($string1 =~ m/l+/) {
  print "There are one or more consecutive l's in $string1\n";
}

$string1 = "Hello World\n";
if ($string1 =~ m/H.?e/) {
  print "There is an 'H' and a 'e' separated by ";
  print "0-1 characters (Ex: He Hoe)\n";
}

$string1 = "Hello World\n";
if ($string1 =~ m/(l+?o)/) {
  print "The non-greedy match with one or more 'l' ";
  print "followed by an 'o' is 'lo', not 'llo'.\n";
}

$string1 = "Hello World\n";
if ($string =~ m/el*o/) {
  print "There is a 'e' followed by zero to many";
  print "'l' followed by 'o' (eo, elo, ello, elllo)\n";
}

$string1 = "Hello World\n";
if ($string1 =~ m/l{1,2}/) {
 print "There exists a substring with at least 1";
 print "and at most 2 l's in $string1\n";
}

$string1 = "Hello World\n";
if ($string1 =~ m/[aeiou]+/) {
  print "$string1 contains a one or more";
  print "vowels\n";
}

$string1 = "Hello World\n";
if ($string1 =~ m/(Hello|Hi)/) {
  print "Hello or Hi is ";
  print "contained in $string1";
}

$string1 = "Hello World\n";
if ($string1 =~ m/\bllo\b/) {
  print "There is a word that starts with";
  print " 'llo'\n";
} else {
  print "There are no words that start with";

  print "'llo'\n";
}

$string1 = "Hello World\n";
if ($string1 =~ m/\w/) {
  print "There is at least one alpha-";
  print "numeric char in $string1 (A-Z, a-z, 0-9, _)\n";
}

$string1 = "Hello World\n";
if ($string1 =~ m/\W/) {
  print "The space between Hello and ";
  print "World is not alphanumeric\n";
}

$string1 = "Hello World\n";
if ($string1 =~ m/\s.*\s/) {
  print "There are TWO whitespace ";
  print "characters separated by other characters in $string1";
}

$string1 = "Hello World\n";
if ($string1 =~ m/\S.*\S/) {
  print "There are TWO non-whitespace ";
  print "characters separated by other characters in $string1";
}

$string1 = "99 bottles of beer on the wall.";
if ($string1 =~ m/(\d+)/) {
  print "$1 is the first number in '$string1'\n";
}

Output:
99 is the first number in '99 bottles of beer on the wall.'

$string1 = "Hello World\n";
if ($string1 =~ m/\D/) {
  print "There is at least one character in $string1";
  print "that is not a digit.\n";
}

$string1 = "Hello World\n";
if ($string1 =~ m/^He/) {
  print "$string1 starts with the characters 'He'\n";
}

$string1 = "Hello World\n";
if ($string1 =~ m/rld$/) {
  print "$string1 is a line or string";
  print "that ends with 'rld'\n";
}

$string1 = "Hello World\n";
if ($string1 =~ m/[^abc]/) {
  print "$string1 does not contain the characters a, b, and c\n";
}

Note that the 'm' in the above regular expressions, for example m/[^abc]/ is not necessary for perl to recognize the expression as type 'match' (as opposed to 'substitute', or s/a/b/), one could just as well use /[^abc]/ without the preceeding 'm'. The 'm' operator can be used to alter the delimiting character, for example m|/| can be used to enhance the readability of /\//. See 'perldoc perlre' for details.

In common with C, obfuscated code competitions are an interesting feature of the Perl culture. Similar to obfuscated code but with a different purpose, Perl Poetry is the practice of writing poems that can actually be compiled by perl. This practice is fairly unique to Perl, due to the large number of regular English words used in the language. New poems can regularly be seen here .

Name

"Perl" was originally named "pearl", after the "pearl of great price" of Matthew 13:46. Larry Wall wanted to give the language a short name with positive connotations and claims he looked at (and rejected) every three- and four-letter word in the dictionary. He even thought of naming it after his wife Gloria.

Before the language's official release Wall discovered that there was already a programming language named "pearl", and changed the spelling of the name. Several backronyms have been suggested, including the humorous Pathologically Eclectic Rubbish Lister. Practical Extraction and Report Language has prevailed in many of today's manuals, including the official Perl man pages. The name is normally capitalized ("Perl") when referring to the language, and uncapitalized ("perl") when referring to the interpreter program itself. (There is a saying in the Perl community that "Nothing but perl can parse Perl".)

Perl Humor

• Perl humour on wikibooks
• State of the Onion 2003 (Larry Wall on Perl 6)
• Larry Wall quotes
• Lingua::Romana::Perligata - Write Perl in Latin!
• Perl Purity Test

See also

• Just another Perl hacker
• Obfuscated Perl contest
• POE -- Perl Object Environment
• Fibonacci number program
• Perl poetry

External links

• Perl.com
• dmoz on Perl
• Perl.org
• Perl Mongers user group site
• The Perl Monastery
• ActiveState - Perl for Microsoft Windows platforms
• IndigoStar , home of IndigoPerl , another distribution of Perl for the Microsoft Windows platform
• CPAN - Comprehensive Perl Archive Network
• Search the Comprehensive Perl Archive Network
• Perl 6 development
• Parrot virtual machine
• Perl POD documentation
• Scripting on the Lido Deck by Steve Silberman, Wired Magazine article about Perl Whirl 2000
• Interview with Larry Wall on Perl (May 01, 1999)
• Perl Timeline
• First reference to "Perl" on Usenet

Books

• Programming Perl (often called the Camel book) ( read online )
• Perl Cookbook (read online: 2nd edition 1st edition ).
• Learning Perl (also called the Llama book)