RegExr
My favorite is <a href="https://regex101.com/" rel="nofollow">https://regex101.com/</a><p>But I've found all of the following to be useful:<p><a href="http://regexone.com/" rel="nofollow">http://regexone.com/</a><p><a href="http://www.regexpal.com/" rel="nofollow">http://www.regexpal.com/</a><p><a href="http://www.rexegg.com/" rel="nofollow">http://www.rexegg.com/</a><p><a href="http://regexr.com/" rel="nofollow">http://regexr.com/</a><p><a href="https://www.debuggex.com/" rel="nofollow">https://www.debuggex.com/</a><p><a href="https://regexper.com/" rel="nofollow">https://regexper.com/</a><p><a href="https://www.regular-expressions.info/" rel="nofollow">https://www.regular-expressions.info/</a><p><a href="http://www.cheatography.com/davechild/cheat-sheets/regular-expressions/" rel="nofollow">http://www.cheatography.com/davechild/cheat-sheets/regular-e...</a><p><a href="http://eloquentjavascript.net/09_regexp.html" rel="nofollow">http://eloquentjavascript.net/09_regexp.html</a><p><a href="http://www.smashingmagazine.com/2009/06/essential-guide-to-regular-expressions-tools-tutorials-and-resources/" rel="nofollow">http://www.smashingmagazine.com/2009/06/essential-guide-to-r...</a><p><a href="http://www.smashingmagazine.com/2009/05/introduction-to-advanced-regular-expression" rel="nofollow">http://www.smashingmagazine.com/2009/05/introduction-to-adva...</a><p><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions" rel="nofollow">https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guid...</a><p><a href="http://codylindley.com/techpro/2013_05_14__javascript-regular-expression-/" rel="nofollow">http://codylindley.com/techpro/2013_05_14__javascript-regula...</a>