It should be a simple question: What are the differences between the "Perl-compatible regular expressions" (PCRE), supported by many languages, and Javascript regular expressions?
Sites like regexpal.com offer a choice between the two, but I'm not sure what the differences are. A quick google produced no results, and this question looks like it should be a duplicate but is actually about matching email addresses-- no help at all, except to confirm that there are differences.
I'd be happy with a reference to a site that documents the peculiarities or limitations of multiple regexp implementations, as long as these two are included.