regex for alphanumeric, but at least one character

Question

In my asp.net page, I have an input box that has to have the following validation on it:

Must be alphanumeric, with at least 1 character (i.e. can\'t be ALL numbers)

Answer 1

Most answers to this question are correct, but there's an alternative, that (in some cases) offers more flexibility if you want to change the rules later on:

^(?=.*[a-zA-Z].*)([a-zA-Z0-9]+)$

This will match any sequence of alphanumerical characters, but only if the first group also matches the whole sequence. It's a little-known trick in regular expressions that allows you to handle some very difficult validation problems.

For example, say you need to add another constraint: the string should be between 6 and 12 characters long. The obvious solutions posted here wouldn't work, but using the look-ahead trick, the regex simply becomes:

^(?=.*[a-zA-Z].*)([a-zA-Z0-9]{6,12})$