perlintro -- a brief introduction and overview of Perl
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 [Toc] [Back]
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 [Toc] [Back]
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 ("0):
print "Hello, $name0; # works fine
print 'Hello, $name0; # prints $nameliterally
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, world0);
print "Hello, world0;
More detailed information about Perl syntax can be found
in perlsyn.
Perl variable types [Toc] [Back]
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 predeclare
your variable types.
Scalar values can be used in various ways:
print $animal;
print "The animal is $animal0;
print "The square of $answer is ", $answer * $answer, "0;
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'}0;
Exhaustive information on the topic of references can be
found in perlreftut, perllol, perlref and perldsc.
Variable scoping [Toc] [Back]
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 $a = "foo";
if ($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 $b" would cause a compile-time
error and prevent you from running the program.
Using "strict" is highly recommended.
Conditional and looping constructs [Toc] [Back]
Perl has most of the usual conditional and looping constructs
except for case/switch (but if you really want it,
there is a Switch module in Perl 5.8 and newer, and on
CPAN. See the section on modules, below, for more information
about modules and CPAN).
The conditions can be any Perl expression. See the list
of operators in the next section for information on comparison
and boolean logic operators, which are commonly
used in conditional statements.
if
if ( condition ) {
...
} elsif ( other condition ) {
...
} else {
...
}
There's also a negated version of it:
unless ( condition ) {
...
}
This is provided as a more readable version of "if
(!condition)".
Note that the braces are required in Perl, even if
you've only got one line in the block. However, there
is a clever way of making your one-line conditional
blocks more English like:
# the traditional way
if ($zippy) {
print "Yow!";
}
# the Perlish post-condition way
print "Yow!" if $zippy;
print "We have no bananas" unless $bananas;
while
while ( condition ) {
...
}
There's also a negated version, for the same reason we
have "unless":
until ( condition ) {
...
}
You can also use "while" in a post-condition:
print "LA LA LA0 while 1; # loops forever
for Exactly like C:
for ($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 $_0;
}
# you don't have to use the default $_ either...
foreach my $key (keys %hash) {
print "The value of $key is $hash{$key}0;
}
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 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 .= "0; # same as $a = $a . "0;
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.0;
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
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
a whitespace character (space,
tab, newline)
non-whitespace character
a digit (0-9)
a non-digit
0 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:
/^+/ string starts with one or more
digits
/^$/ nothing in the string (start
and end are adjacent)
/({3}/ a three digits, each followed by a
whitespace
character (eg "3 4 5 ")
/(a.)+/ matches a string in which every odd-numbered letter
is a (eg "abacadaf")
# This loop reads from STDIN, and prints non-blank
lines:
while (<>) {
next if /^$/;
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 $10;
print "Hostname is $20;
}
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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.
Kirrily "Skud" Robert <[email protected]>
perl v5.8.5 2002-11-06 13 [ Back ] |
