How to use negated character classes [^...] in Perl regex?

The negated character class [^...] in Perl regular expressions is a powerful way to match any character except those listed inside the square brackets. It is the inverse of a normal character class, which matches characters that are in the set.

What is a negated character class?

Inside a regex, a character class is defined by square brackets []. For example:

• [aeiou] matches any lowercase vowel character a, e, i, o, u.
• [^aeiou] matches any character except these vowels.

The caret ^ right after the opening bracket [ acts as a negation operator, meaning "match anything but these characters."

How to use negated character classes in Perl regex?

To use a negated class, just include the caret as the first character inside the brackets, followed by the characters or character ranges to exclude. Here are some examples:

• [^a-z] matches anything that is not a lowercase letter.
• [^0-9] matches any non-digit character.
• [^ \t\n] matches any character except space, tab, or newline (i.e., non-whitespace if you define whitespace as these).

Perl-specific notes:

• Character classes can include ranges, e.g., [a-zA-Z] matches letters.
• TMTOWTDI (There's More Than One Way To Do It) applies — you can achieve similar negation using other regex constructs, but [^...] is often the simplest.
• Be careful with meta-characters inside the class; for example, to include a literal ], you may need to place it first or escape it.
• Negation only applies inside the brackets; ^ outside has a different meaning (start of line).

Common Pitfalls

• Using ^ anywhere but right after the opening bracket does not negate inside the class.
• Remember that negated classes still match one character - they do not mean 'match zero or more non-matching chars' unless combined with quantifiers ([^a-z]+).
• Be mindful of Unicode or locale settings that affect character ranges like a-z.

Example Perl Code

The following code demonstrates the practical use of negated character classes by extracting sequences of non-digit characters from a string:

#!/usr/bin/perl
use strict;
use warnings;

my $string = "abc123XYZ!@#456def";

# Match runs of characters that are NOT digits (\d)
# [^0-9]+ means match one or more characters other than 0-9
while ($string =~ /([^0-9]+)/g) {
    print "Matched non-digit substring: '$1'\n";
}

# Another example: match strings without vowels
while ($string =~ /([^aeiouAEIOU\s]+)/g) {
    print "Matched substring without vowels: '$1'\n";
}

Output:

Matched non-digit substring: 'abc'
Matched non-digit substring: 'XYZ!@#'
Matched non-digit substring: 'def'
Matched substring without vowels: 'bc'
Matched substring without vowels: '123XYZ!@#456df'

Here, the first regex uses [^0-9] to find all sequences of non-digit characters. The second example uses [^aeiouAEIOU\s] to match chunks that contain no vowels or whitespace. This shows how negated classes give you control over what is excluded from matches.

Summary

• Use [^...] to match any character except those listed inside the brackets.
• Place the caret ^ immediately after the opening bracket to denote negation.
• Combine with quantifiers (+, *, etc.) to match sequences.
• Beware of special characters and ensure correct placement of ^.
• Negated character classes are a core regex feature in Perl and work as expected across Perl 5 versions.