Chris Shiflett would be proud of the latest post over on Dynamically Typed today titled "Validate Your Input!" in which he attempts to explain what validation is and why it is important.

I remember being taught the importance of input validation way back in high school, in an IT elective subject. We were taught that all user input must be validated prior to acting upon it. At that stage we were 'programming' in Microsoft Excel using VBscript, but validation is all the more relevant for web applications, which may be used - or exploited - by a wide range of people.

Data validation is the process of checking user input to see whether it is in the expected format and is within the range of allowed values for that input.

He also gives a few sample ways that you can validate different variables forms, including strings, numeric values, boolean, and enumerated values. Regular expressions can play a big part in validation, so, if you haven't gotten a chance to get into them, it's never to late to start...