(Obligatory relevant repost:) My entry into this category: http://regex.powertoy.org/ http://regex.powertoy.org/ Important caveat: it makes use of a hidden J

(Obligatory relevant repost:) My entry into this category: http://regex.powertoy.org/ http://regex.powertoy.org/ Important caveat: it makes use of a hidden Java applet -- so that it can supports the somewhat larger Java regex syntax, doesn't send your data anywhere else for matching, and can hook into the string-probing to animate the process. So dig out whatever browser you use for Java applets (if you still have one) to test. Regarding the animation, click the 'animate?' link to show the animation step/speed controls. For example, you can watch the regex that tests whether a number is prime (by failing) or composite (by succeeding) via these two animations: 49: http://regex.powertoy.org/?pat=/^1%3F%24|^%2811+%3F%29\1+%24.. http://regex.powertoy.org/?pat=/^1%3F%24|^%2811+%3F%29\1+%24.... 47: http://regex.powertoy.org/?pat=/^1%3F%24|^%2811+%3F%29\1+%24.. http://regex.powertoy.org/?pat=/^1%3F%24|^%2811+%3F%29\1+%24.... I really want to get rid of the applet requirement; I might someday cross-compile the JDK7 regex support to JS so that the full syntax and animation can still be supported, without an applet.