How does the '[^][]' regular expression component work in matching nested square brackets?

What Does the "[^][]" Regex Mean?

In the provided regex:

\[(?:[^][]|(?R))*\]

the "[^][]" regex component is a character class that matches any character except "[" or "]".

Character Class Explanation

A character class matches any character within the given range of characters. In this case, "[^]" means "not followed by ]", so "[^][]" effectively matches any character that is not followed by "]".

Regex Recursion

The parentheses around "[^][]" indicate a non-capturing group. Inside the group, the "?R" token represents a recursive reference to the entire regex. This allows the regex to match nested square brackets.

Avoiding Escape Sequences

Note that, in PCRE (the regex engine used by PHP's preg_ functions), it is not necessary to escape "[" or "]" within a character class. This is because it is unambiguous that the "]" within "[^][]" belongs to the character class.

Inline xx Modifier (PHP 7.3 )

In PHP 7.3 , you can use the inline xx modifier to ignore blank characters within character classes. This allows you to write the following less ambiguous classes:

(?xx) [^ ][ ]     [ ] ]      [ [ ]      [^ [ ]

Compatibility and Quirks

The "[^][]" syntax is compatible with most regex flavors, including PCRE, Perl, Python, Java, and others. However, it is not recognized in Ruby and JavaScript (except in older versions of Internet Explorer).

Nested Square Bracket Matching

In the context of your sample regex, "[^][]" ensures that balanced square brackets are matched, allowing for the matching of nested square brackets.

Additional Notes

• "[^]]" is unambiguous because "[^]" matches characters not followed by "]", so the subsequent "]" belongs to the character class.
• Modern JavaScript browsers generally follow the ECMA specification, which defines "[]" as a regex token that always fails to match.
• The provided optimized regex "([[^][](?:(?-1)[^][])* ])" improves performance by avoiding unnecessary alternations.

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