Help for a form newbie.

I need to verify input to a text box in a form.
It must be exactly 6 characters.
First is always "p"
The next 5 are any number.

I know how to verify if it is all a number but not sure
how to do it for a combo of letter and numbers.

Any help would be appreciated. Sorry if this is a second
post. I posted yesterday but did not see it appear.

Thanks in advance