Regex Tester & Validator

Matches the classic context-free language a^n b^n: strings with exactly n 'a's followed by n 'b's, including the empty string. It uses a recursive subroutine call (?1) to match balanced a/b pairs, and the x flag allows whitespace in the pattern. Useful for validating equal-count pair constructs or demonstrating recursive regex capabilities in PCRE.