Regular Expression ...

Question

I would like to validate how many characters a user types in a textbox. So, I'm trying to use a regular expression to do it. And I thought it would be easy. But it isn't I think. \w{9}$ works up to 9 character, but \w{10}$ doesen't wort at all! How can I write an expression to validate let's say a maximum of 40 characters i a textbox?