Ok, i really need help making something like this, Basically i sell stuff on my server, the payment methods i use is paypal and i want to be able to make a page before paypal and make text boxes like.

Name:
Adress:
Steam ID:

And stuff in that sort, but i need a way for it to check for errors, like on STEAM ID, i need a way for it to give them a error if that steamid is not in the following format

STEAM_X:X:XXXXXXX Replace X's with #'s and how many #'s can vary. I just need a way for it to check for all STEAM IDs and if people gave me a steam id like "ABCD" It would be incorrect it has to start with STEAM_X:X:XXXXX and all the X's are replaced with #'s and can vary. Once thats done, i still want it to be able to send the $$ to my paypal. After the purchase is done, i want it to like be able to login to my FTP for a server of mine, and change a file by adding

STEAM_X:X:XXXXX <-- The steam id they used above to purchase onto the file, and add a few letters behind it for example.

STEAM_X:X:XXXXXX ABCDEFG. One last thing, above it, it would print their next payment due. As the paypal method im using is a subscription so it would print 1 month from the date of purchase. Something like, if they purchased today Dec 3. It would write this into the file

//January 3, 2007
STEAM_X:X:XXXXXXX ABCDC

And it has to leave the //