Validate Textbox

Question

Hello,

 

        Validate  text box that accept only Digits, Letters and First charecter should be Capital P only and the Maximum length of Textbox should be 6 charecters.  

i know the code to accept digits and letters and Max length but i don't know how to set First charecter of Text box to capital P. pls some one provide me code ..

 

 

Thanks