SQL 2000.

I need a query that will search a field in the table that is 14
characters long and begins with a number.

I have a client that was allowing people to store credit card numbers,
plain text, in an application. I need to rip through the table and
replace every instance of credit card numbers with "x" and the last 4
digits. I got the replace bit going, but I am stuck on how to search
for a string in a field of a specific length.

Any ideas?

Thanks,
--Mike