Perl's regex support can match HTML because of recursive matching, not only because of backreferences (the latter of which is widely implemented, the former not

Perl's regex support can match HTML because of recursive matching, not only because of backreferences (the latter of which is widely implemented, the former not quite so). That being said, there surely are some fun languages that can be matched by what's commonly called regular expressions. Notepad++ was notable (before switching to PCRE) that its "regular expressions" could not even match every regular (or even finite) language (http://stackoverflow.com/a/4815422/73070 http://stackoverflow.com/a/4815422/73070). Many regex engines allow matching languages that are context-sensitive, while at the same time not accepting all context-free languages.