Matches

Returns true when the string value of matchSource matches pattern, with XPath fn:matches semantics - unanchored (a substring match); authors write ^...$ for a whole-string match. The pattern language is the pinned RE2-compatible XSD-regex subset defined by the expression runtime; a pattern outside the subset is a loud evaluation error, never a silent false, and the subset excludes every construct that can backtrack catastrophically, so evaluation cost is linear in the input. A non-string matchSource is an error (a computation type confusion, not a falsified predicate).