How to restrict usernames to alphanumeric

there is a field named "List of forbidden names separated by |", the forbidden names here can actually be a general regular expression.

For our problem, we don't want any characters that is not a digit or alphabet character, the pattern for this is:

[^a-zA-Z0-9]

Explanation:
[ ]: used to express a set of characters
a-zA-Z0-9 inside [] means characters 0 to 9, a to z and A to Z
^ at the begining means NOT.