How does the '[^][]' Regular Expression Work in PCRE?

Understanding the "[^][]" Regular Expression:

The regular expression "[^][]" in the provided pattern "[(?:[^][]|(?R))*]" matches all characters that are not square brackets ([ or ]).

Explanation:

Unique Considerations:

Unlike other regex engines, PCRE (used in PHP's "preg_" functions) does not require escaping [ or ] within a character class. This is because PCRE distinguishes them from other character classes.

In addition, the regex pattern "[^]]" is not ambiguous because ] is the first character. This means it matches characters followed by a ]. To match an a followed by ], write ¹.

Optimizing the Pattern:

To make the pattern more reusable and efficient, it can be optimized as follows:

• ([(?:[^][] |(?-1))* ]): This pattern captures square brackets with nested brackets.
• ([[^][](?:(?-1)[^][])* ]): This pattern is even more efficient by eliminating the alternation.

Variations across Programming Languages:

The inline "xx" modifier, introduced in PHP 7.3, allows whitespace to be ignored within character classes. However, this syntax is not supported by all regex flavors, such as Ruby and JavaScript. In JavaScript, specifically, "[]" is a token that always fails to match, and "[^]" matches any character.

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