What does the character class '[^][]' do within a regular expression?

Decoding the "[^][]" Character Class

Within the regex [(?:[^][]|(?R))*], the character class "[^][]" raises a peculiar question. Let's elucidate its meaning and dispel any ambiguity.

Meaning of "[^][]"

"[^][]" is a character class that matches any character except "[" and "]". This expression effectively excludes "[" and "]" from the set of matching characters.

Ambiguity Resolution

Some may argue that the sequence "[^]" is ambiguous, leading to a possible interpretation as an escape for "^" and a literal "]". However, PCRE, the regex engine utilized by preg_ functions, resolves this ambiguity. Since "[^]" is invalid in PCRE, the parsing interprets "]" as being enclosed within the character class, leaving the class to be closed by a subsequent "]".

Additional Contextual Rules

• "[^]" can be written as "¹" or "[^][]" without escaping the "[" or "]".
• The inline "xx" modifier in PHP 7.3 eliminates the need for escaping even within character classes.
• The first character in the character class determines the interpretation. "[^]]" matches any character followed by "]", while "²]" matches a non-"a" followed by "]".
• In JavaScript, "[]" is defined as a token that never matches, while "[^]" matches any character.
• The provided regex pattern [(?:[^][]|(?R))*] matches square brackets, allowing recursion for nested brackets.
• A more efficient and reusable pattern for matching brackets is "([[^][](?:(?-1)[^][])* ])".

Conclusion

The character class "[^][]" simply excludes the characters "[" and "]" from matching. This interpretation is consistent across PCRE-compatible engines, including JavaScript (with some exceptions). By understanding these rules, programmers can effectively utilize this character class in their regex patterns.

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