So, yes, some implementations of regular expressions are Turing complete because they run arbitrary code, but that is rather the opposite of a way to make parse

So, yes, some implementations of regular expressions are Turing complete because they run arbitrary code, but that is rather the opposite of a way to make parsers safer. On that note, once you can run arbitrary functions on your matches, you could match /.*/ and then the function you run is html5lib.parse. Is that still a regular expression?