Perl Regular Expressions

Regular expressions (regex) describe a pattern for matching characters in strings. They can be used to check if a string contains a certain substring, replace matching substrings, or extract substrings that meet certain criteria.

Perl's regular expression capabilities are extremely powerful, often considered the most robust among commonly used languages. Many languages that support regular expressions reference Perl's implementation.

Perl's regular expressions come in three forms: matching, substitution, and translation:

• Matching: m/
• Substitution: s/
• Translation: tr/

These forms are typically used with =~ or !~, where =~ denotes a match and !~ denotes no match.

Matching Operator

The matching operator m// is used to match a string or a regular expression. For example, to match "run" in the scalar $bar, the code would look like this:

Example

#!/usr/bin/perl

$bar = "I am tutorialpro site. welcome to tutorialpro site.";
if ($bar =~ /run/){
   print "First match\n";
}else{
   print "First no match\n";
}

$bar = "run";
if ($bar =~ /run/){
   print "Second match\n";
}else{
   print "Second no match\n";
}

Executing the above program will output:

First match
Second match

Pattern Matching Modifiers

Some common pattern matching modifiers are:

Regular Expression Variables

After processing, Perl stores the matched values in three special variables:

• $: The part of the string before the matched portion
• $&: The matched string
• $': The part of the string after the matched portion

Combining these variables will give you the original string.

Example

#!/usr/bin/perl

$string = "welcome to tutorialpro site.";
$string =~ m/run/;
print "String before match: $`\n";
print "Matched string: $&\n";
print "String after match: $'\n";

Executing the above program will output:

String before match: welcome to 
Matched string: run
String after match: oob site.

Substitution Operator

The substitution operator s/// extends the matching operator, replacing the specified string with a new one. The basic format is:

s/PATTERN/REPLACEMENT/;

Where PATTERN is the matching pattern and REPLACEMENT is the string to replace with.

For example, to replace "google" with "tutorialpro" in the following string:

Example

#!/usr/bin/perl

$string = "welcome to google site.";
$string =~ s/google/tutorialpro/;

print "$string\n";

Executing the above program will output:

welcome to tutorialpro site.

Substitution Modifiers

Substitution modifiers include:

Translation Operator

Modifiers related to the translation operator include:

The following example converts all lowercase letters in the variable $string to uppercase:

#!/usr/bin/perl

$string = 'welcome to tutorialpro site.';
$string =~ tr/a-z/A-Z/;

print "$string\n";

Executing the above program will output:

WELCOME TO TUTORIALPRO SITE.

The following example uses /s to remove duplicate characters in the variable $string:

Example

#!/usr/bin/perl

$string = 'tutorialpro';
$string =~ tr/a-z/a-z/s;

print "$string\n";

Executing the above program will output:

runob

More examples:

$string =~ tr/\d/ /c;     # Replace all non-digit characters with a space
$string =~ tr/\t //d;     # Remove tabs and spaces
$string =~ tr/0-9/ /cs    # Replace other characters between digits with a space

More Regular Expression Rules

More Reference Content

Regular Expressions: https://www.tutorialpro.org/regexp/regexp-tutorial.html

Perl Regular Expressions: https://perldoc.perl.org/perlre#Regular-Expressions