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/
    }
}

Sure you could write an irule to do this, have an irule event trigger it, store the result in a session variable, and then take a branch path to deny based on the result variable. If you really wanted to you could even use an Empty box in the VPE and put all the TCL into the branch selection, but that might get a little hairy as you have to write it all on one line.