Take for example, the following regular expression. Can you tell what it is doing? If so... then great, but what about the developers you work with?
Which code sample would you rather maintain? And more importantly which one would your co-worker be more likely to understand?
NOTE: The above email regular expression is very naive in it's logic. I based the above snippet from a Nettuts+ post entitled, 8 Regular Expressions You Should Know. There are much more comprehensive email regular expressions available on the internet, but I used the above one to show the value of annotation.
As a side note, some tools that I find helpful are Grant Skinner's Online RegExr Tool and I sometimes get inspiration for regular expressions at RegExLib.com. What tools or resources do you use for regular expressions?