Regular expression not working with WAF rules

I am trying to add a custom rule to block a few requests matching the URL path via regular expression but it seems not working.

My requirement is if URL begins with “/admin/” then it should be blocked. Also if someone tries to add any character between the “/” other than “admin” then it should be blocked and it should have a case insensitive match.

I am using the following condition with my rule -

(http.request.uri.path matches “(?i)^/(\b)(admin)(\b){1}/”)

Few scenarios are not working with the above regular expression (expectation is following URL should also be blocked). For e.g.

/aDmin/a.txt
/admin…/a.txt
/admin%2e%2e/a.txt
/aDMin/a.txt

Could someone help me to understand what is wrong with my regular expression?

Yes, we are on business plan.