docdawson:

 ^(\w+([-+.]\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*){n,20}$

Doesn't this just match n to 20 e-mail addresses? I understand carlosaml wants addresses with no more than 20 characters.

The shortest possible address that will match the pattern above has 5 characters (a@b.c).

To check for 5 to 20 characters one would have to use a lookahead assertion:

^(?=.{5,20}$)\w+([-+.]\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*$