A regular expression is a powerful tool for matching or manipulating strings. You might be unfamiliar with the term but I bet you have used a regular expression before. Most search and replace tools in text editors use regular expressions to match the text you are looking for and replace it with the text you specify. Using regular expressions might not be that hard but understanding the syntax for writing your own can be quite a chore.
Lucky for us Design215.com has an excellent regular expression testing tool complete with a list of syntax properties and detailed examples. Just construct your regular expression, enter some input text to test against and hit the test button. The output code will tell you if your regular expression worked or not so you can make further tweaks.
This little tool can save a lot of time when you need to develop a regular expression for your code.