Haven't you heard of a check constraint?

If I recall correctly, postgresql has some support for regular expressions:
http://www.postgresql.org/docs/8.1/static/functions-matching.html

Does it support general constraints with a CHECK keyword? Yes it does:
http://www.postgresql.org/docs/8.1/static/ddl-constraints.html

Check constraints are light-weight when it comes to defining domains. You
need to enforce this rule (not a domain)  via a trigger or a stored proc.
And still you can join a price with a weight, which destroys the whole idea
of a domain.

Evan

PostgreSQL was designed explicitly to handle this kind of problem.

Have a look at the entire manual section here:

http://www.postgresql.org/docs/8.1/static/extend.html

with particular attention to:

http://www.postgresql.org/docs/8.1/static/xtypes.html

and the few sections following.

What you want to do here is to define a new relational 'domain' (note:
NOT a SQL DOMAIN, which is simply a renaming of another base type).
Validity checking is done (mostly) when queries are parsed.

I am also going to guess that your new 'data type/DOMAIN' has other
requirements, such as ordering. Does "AA123" come before or after
"12345", for example?

You can define your new data type / domain and any rules that you wish
to apply over it in 'C', PL or Tcl.