hello friends.

i am php programmer, i want to do js validation with my form.
in details like

!)i have product id, and that wanted to be an alphanumeric.
that is working perfectly fine with js.

2)but i want some formated alphanumeric value
e.g.
prod_id=L28
means first character should be alphabet and after i want only two digit

may be its too simple but i am new comer.
so please help me.
thanks in advance