Regex Tester & Validator

This regular expression uses a positive lookahead to assert that the text immediately following the current position is exactly "perro" (the Spanish word for dog). Because it is a zero-length assertion, it matches a specific position rather than consuming the actual characters. When used with the global and multiline flags, it will successfully match at the starting position of every occurrence of "perro" throughout the text without advancing the match index past the word itself.