As originally demoed at Bar Camp last August, Regex Powertoy lets you interactively tune a regular expression inside a web browser. This version fixes a number of glitches and adds many new capabilities, including:
Perl-style syntax option (now the default)
Replacements with backreferences
Highlighting alternatives: emphasize matches, edits, replacements, or splits
Improved capturing group detail display
Bookmark patterns and settings to a 'matchmark'
Regex syntax reminder help
All matching is done locally by an applet -- there's no network traffic after the page loads.
Regex Powertoy was developed in Firefox with Java 5 (aka 1.5) applet support, but mostly works in IE6, and should work with Java 2 (aka 1.4).
Comments, suggestions, bug reports, and patches are all welcome!