Regular expressions are an incredibly powerful tool, but can be rather tricky to get exactly right. This is a website that I wrote so I could quickly and easily test regular expressions during development.
Pick which programming language you are using:
Share codes are an easy way to share regexes. If you have a share code, enter it here:
A badly formed regular expression can run forever, making it very useful for denial of services attacks. Some of the engines/hosts are susceptible to these. Check the current engine status to see if they are running.