As for XRegExp, it has recently been upgraded to v0.5.2, which resolved a corner-case bug involving
XRegExp.matchRecursive. See the changelog for details.
I'll take this opportunity to highlight some of my other favorite online regex testers. I've actively looked for these kinds of apps over the years and have seen more than 50 of them. Odds are you'll find something new here.
Edit (2008-06-18): Updated the list with a couple that have come out very recently.
- Regexp Editor — Java regex tester with regex syntax highlighting.
- RegExr — ActionScript regex tester with regex syntax highlighting.
- reAnimator — Fun app for visualizing regex FSAs.
- The REWizard — IE only, but offers regex building tools and an interesting visualization.
- MyRegexTester — Includes code generation and plain-text explanations (via the YAPE::Regex::Explain Perl module).
- Nregex — .NET regex tester.
- Rubular — Ruby regex tester.