F5 APM - iRule reviewing pattern matching

I have not tested the following but it will give you something to get started with:

when ACCESS_ACL_ALLOWED {
    if {[string tolower [ACCESS::session data get "session.logon.last.username"]] matches_regex {^[A-Za-z]{3}\d{4}$} } {
        HTTP::redirect http://my.other.place/
    }
}