CHAR to INT

I'm trying to make a PL function to pass from an alphanumeric string
to a numeric one changing the chars in the string to it's ORD (the
inverse of the chr function) and I'm stumbling with two problems:

1) How to know if the CHAR is numeric or not.
2) If it's not numeric I want to get it's ord, but I can't find
something like that in PG.

Any ideas?

--
Martín Marqués
select 'martin.marques' || '@' || 'gmail.com'
DBA, Programador, Administrador