enforcing alphabetic/numeric content in "char" column

Hello,

The task is to allow data entries like "007", leading zeros
included. I tried various numeric data types and they all trim
it down to "7". Using char(3) works, but I want to put a CHECK
on it to ensure no non-numeric data is entered. The following,
and numerous variations on it, do not work:

iso3dnum    char(3) CHECK (iso3dnum ~ '[0-9]')

Any suggestions ?

Cheers,
Joel