Perl has probably the most complete regular expression set of any
computer language. To see with your eyes how perverse, er, powerful,
they can be, check out the longest regular expression ever seen by
humans at www.ex-parrot.com/~pdw/Mail-RFC822-Address.html. Rumours are
that it validates email addresses, but don't look at it for too long.
To write your own instead, the ultimate reference is "Mastering Regular
Expressions" by J. Friedl (www.oreilly.com/catalog/regex2/).