Prohibited User REGEX does not work

I have created and validated a regex to identify prohibited user names. The regex does not seem to be executed as I am still getting registration requests with user names that should have been identified as prohibited. What am I doing wrong.

Here is the regex as placed in the edit box:
/^(?!(xxxx)|(yyyyy)|:alpha:{1, 2}[0-9]).*$/

On another note, WP Cerber features indicate that is should be able to “Restrict user registration or login with a username matching REGEX patterns” however I cannot find any documentation or page that allows for configuring this. Only “Prohibited username”

Thank you in advance
Paula