Those of you who can read Regular Expressions as easily as written text can skip this post. But for the rest of us ...
I mentioned a while back that this Regular Expression Analyzer was a great help debugging a Regex, esp. one originally written by someone else.
Thought I should note a new one I stumbled on yesterday Regular Expression 101. Functionality is equivalent, but seems a clearer layout with a cheat sheet on the same page too. The real advantage is that a Regex test match tool is integrated in!
Both have the ability to share a Regex: the myezapp one passes it all as url parameters, but the 101 one also allows test cases to be saved and uses a tiny url.
As an example here's one I was working on yesterday, to validate a url path only to an image (ie the actual filename had already been stripped). Goal was to allow dots in the domain name, but not in any of the directories - to avoid a url like www.domain.tld/malware.php followed by a /filename.jpg