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Catalog / Perl Programming Cheatsheet

Perl Programming Cheatsheet

A concise reference for Perl syntax, built-in functions, regular expressions, and common programming idioms. This cheat sheet provides a quick guide to the essential elements of Perl for both beginners and experienced programmers.

Basic Syntax and Data Types

Variables

Scalars

Start with $.

$name = "John";
$age = 30;

Arrays

Start with @.

@names = ("John", "Jane", "Doe");

Hashes

Start with %.

%ages = (
  "John" => 30,
  "Jane" => 25,
  "Doe"  => 40
);

Variable Interpolation

Variables are interpolated in double-quoted strings.

print "Name: $name, Age: $age\n";

Context

Perl uses scalar, list, and void context to determine how expressions are evaluated.

Operators

Arithmetic

+, -, *, /, %, ** (exponentiation)

String

. (concatenation), x (repetition)

Comparison

Numeric: ==, !=, <, >, <=, >=
String: eq, ne, lt, gt, le, ge

Logical

&&, ||, !, and, or, not

Assignment

=, +=, -=, *=, /=, .=

Control Structures

Conditional Statements:

if ($age >= 18) {
  print "Adult\n";
} elsif ($age >= 13) {
  print "Teenager\n";
} else {
  print "Child\n";
}

unless ($name) {
  print "Name is undefined\n";
}

Loops:

for ($i = 0; $i < 10; $i++) {
  print "$i ";
}
print "\n";

while ($age < 60) {
  $age++;
  print "Age: $age\n";
}

foreach $name (@names) {
  print "Name: $name\n";
}

Loop Control:
next (continue), last (break), redo

Subroutines and Modules

Subroutines

Defining a Subroutine

 
sub greet {
  my ($name) = @_;
  print "Hello, $name!\n";
}

Calling a Subroutine

 
greet("World");

@_

Array containing arguments passed to the subroutine.

Return Values

Subroutines can return values using the return keyword.

sub add {
  my ($x, $y) = @_;
  return $x + $y;
}

Local Variables

Use my to declare local variables within a subroutine scope.

Modules

Using Modules

 
use Module::Name;

Creating Modules

Create a .pm file with the module name.

package My::Module;

use Exporter qw(import);
our @EXPORT = qw(my_function);

sub my_function {
  print "Module function\n";
}

1;

Exporter

Used to export functions from the module’s namespace.

@EXPORT

Array of function names to export.

Always return 1;

Modules must return true to indicate successful loading.

Common Built-in Functions

print: Prints output.

print "Hello, world!\n";

length: Returns the length of a string.

$length = length("abc"); # $length is 3

substr: Extracts a substring.

$substring = substr("hello", 1, 3); # $substring is "ell"

split: Splits a string into an array.

@words = split(/ /, "hello world"); # @words is ("hello", "world")

join: Joins an array into a string.

$string = join(" ", @words); # $string is "hello world"

Regular Expressions

Basic Matching

Pattern Matching

 
if ($string =~ /pattern/) {
  print "Match found\n";
}

Substitution

 
$string =~ s/old/new/;

Translation

 
$string =~ tr/a-z/A-Z/;

Quantifiers

*

Zero or more times

+

One or more times

?

Zero or one time

{n}

Exactly n times

{n,}

n or more times

{n,m}

Between n and m times

Character Classes

.

Any character except newline

\d

Digit (0-9)

\w

Word character (a-z, A-Z, 0-9, _)

\s

Whitespace character (space, tab, newline)

[abc]

Any of the characters a, b, or c

[^abc]

Any character except a, b, or c

Anchors

^

Start of the string

$

End of the string

\b

Word boundary

Modifiers

i

Case-insensitive

g

Global (match all occurrences)

m

Multiline (treat string as multiple lines)

s

Treat string as single line

x

Ignore whitespace in pattern

File I/O and System Interaction

File Handling

Opening Files

 
open(my $fh, "<", "input.txt") or die "Cannot open file: $!"; # Read
open(my $fh, ">", "output.txt") or die "Cannot open file: $!"; # Write
open(my $fh, ">>", "append.txt") or die "Cannot open file: $!"; # Append

Reading from Files

 
while (my $line = <$fh>) {
  chomp $line;
  print "Line: $line\n";
}

Writing to Files

 
print $fh "Hello, file!\n";

Closing Files

 
close($fh);

$!

Contains the system error message.

Command Line Arguments

@ARGV

Array containing command-line arguments.

foreach $arg (@ARGV) {
  print "Argument: $arg\n";
}

Getopt::Long

Module for parsing command-line options.

use Getopt::Long;

my ($name, $age);
GetOptions(
  "name=s" => \$name,
  "age=i"  => \$age
) or die "Error in command line arguments\n";

print "Name: $name, Age: $age\n";

System Calls

system

Executes a system command.

system("ls -l");

qx// or `

Captures the output of a system command.

my $output = `date`;
print "Date: $output\n";

exec

Replaces the current process with a new one. Terminates the current script after successful execution.

exec("find . -name '*.txt'");

Environment Variables

%ENV

Hash containing environment variables.

print "PATH: $ENV{PATH}\n";
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