I don't know about python, but a lot of languages support PCRE (perl compatible regexp) or at least a large subset of it. PCRE is sometimes less than what curre

I don't know about python, but a lot of languages support PCRE (perl compatible regexp) or at least a large subset of it. PCRE is sometimes less than what current Perl supports but in any case a superset of "traditional" regular expressions. For example there's support for recursive subpatterns [1] which clearly places it into context free territory[2]. Other regular expression engines support similar features, so I'd say that most real world implementations are not regular expressions in the formal sense. [1] PHP manual for PCRE recursive patterns http://www.php.net/manual/en/regexp.reference.recursive.php http://www.php.net/manual/en/regexp.reference.recursive.php [2] Recursive subpatterns allow matching a language of the form (a * n)b(a * n) which is not regular.