'Match' even if regexp has syntax-error? Huh?

I have begun using procmail (3.11pre7) on another ISP (teleport.com) 
and was puzzled by the way procmail handled a broken regexp in a
condition line. The regexp was broken due syntax-error from a missing
')', and procmail reported that in the logfile. Yet procmail still
considered it a 'Match'. Is this usual and appropriate for this
version of procmail?  On another site using 3.11pre4 a broken regexp
won't match. Any have an explanation?

logfile excerpt:
procmail: Invalid regexp "^From: .*\<(dnhunt|wjh|the([+][-a-z0-9.+]+)?@"
procmail: Match on "^From: .*\<(dnhunt|wjh|the([+][-a-z0-9.+]+)?@"

Thanks-
        David