PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1)NAMEperlintro-- 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 perl-
toc.
Throughout this document you'll see references to other
parts of the Perl documentation. You can read that documen-
tation 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 develop-
ment, network programming, GUI development, and more.
The language is intended to be practical (easy to use, effi-
cient, complete) rather than beautiful (tiny, elegant,
minimal). Its major features are that it's easy to use,
supports both procedural and object-oriented (OO) program-
ming, 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
perl v5.8.8 2006-06-30 1
PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1)
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 plat-
forms 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 spe-
cial characters such as newlines ("\n"):
print "Hello, $name\n"; # works fine
print 'Hello, $name\n'; # prints $name\n literally
perl v5.8.8 2006-06-30 2
PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1)
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 func-
tions in Perl, and it's set implicitly by certain loop-
ing constructs.
print; # prints contents of $_ by default
Arrays
An array represents a list of values:
perl v5.8.8 2006-06-30 3
PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1)
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:
perl v5.8.8 2006-06-30 4
PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1)
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 refer-
ences.
my $variables = {
scalar => {
description => "single item",
sigil => '$',
},
array => {
description => "ordered list of items",
sigil => '@',
},
hash => {
description => "key/value pairs",
sigil => '%',
},
};
perl v5.8.8 2006-06-30 5
PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1)
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 vari-
ables 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 $b" would cause a compile-
time error and prevent you from running the program. Using
"strict" is highly recommended.
Conditional and looping constructs
Perl has most of the usual conditional and looping con-
structs except for case/switch (but if you really want it,
there is a Switch module in Perl 5.8 and newer, and on CPAN.
See the section on modules, below, for more information
about modules and CPAN).
The conditions can be any Perl expression. See the list of
operators in the next section for information on comparison
and boolean logic operators, which are commonly used in con-
ditional statements.
if
perl v5.8.8 2006-06-30 6
PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1)
if ( condition ) {
...
} elsif ( other condition ) {
...
} else {
...
}
There's also a negated version of it:
unless ( condition ) {
...
}
This is provided as a more readable version of "if
(!condition)".
Note that the braces are required in Perl, even if
you've only got one line in the block. However, there
is a clever way of making your one-line conditional
blocks more English like:
# the traditional way
if ($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++) {
...
}
perl v5.8.8 2006-06-30 7
PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1)
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
perl v5.8.8 2006-06-30 8
PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1)
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 pre-
cedence 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 "<>" opera-
tor. 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:
perl v5.8.8 2006-06-30 9
PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1)
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 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 docu-
mented 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 per-
lop.
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 mean-
time, here's a quick cheat sheet:
perl v5.8.8 2006-06-30 10
PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1)
. 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 pre-
vious 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, $2
and so on.
# a cheap and nasty way to break an email address up into parts
perl v5.8.8 2006-06-30 11
PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1)
if ($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 perl-
var 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 refer-
ences 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 docu-
mented below.
perl v5.8.8 2006-06-30 12
PERLINTRO(1) Perl Programmers Reference Guide PERLINTRO(1)
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 net-
work 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 com-
mon 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>
perl v5.8.8 2006-06-30 13
