postcode system help

[mark | r]

i am creating a small application that requests the input of a post code
which is checked against a list of valid postcodes i have stored in a
database. my problem is this

its easy to check 2 letter postcodes by validating the first two chars e.g.

hx1 5rs
st4 7rr

but its hard because of the ones that are letter number combinations e.g.

L12 5xh
S01 4TQ

anyone know a good solution for this?

thanks

mark