Everyone who has been into development have at least once run into a problem which is to be solved by using a regular expression. The power of these can totally smash the most complex problem into a piece of cake, but the problem remain that very few people actually understand how these expressions work.
A little while ago I came across a flash-tool in which one can test out regular expressions on a specified text. All you do is to paste an example-text into the text-box in the tool and then you can start typing a regular expression and see what that specific expression will hit in the pasted text.
Easy and fast, use it all the time nowadays. Check it out!
Note: You can also find this link under xds.se > Cheat sheets > Links often used