Only regular expressions[0] can compile to FST's; Perl-style regexes aren't regular.

[0] https://en.wikipedia.org/wiki/Regular_language

That's probably, because the regexes you linked to are Perl-flavoured regexes, whereas this website seems to use the "Unix Extended" variant of regex.

If you want crazy, you should look up RegEx match open tags except XHTML self-contained tags.