Excel 2007 : Change text colour when crossing character limit

Hi,

In a cell when I type a word more than 22 characters the text should turn red.

I used data validation for character limit but not able to construct any formula to turn the text red when it crosses the limit.Don't want any error warning as given by data validation.

I also tried with conditional formatting but no success.

PLEASE HELP !!!!

Sorry if its a repost.

Thanks in advance

Vivek

thanks a lot martin