regexec.c: Fix precedence

regexec.c: Fix precedence

Commit ac51e94be5daabecdeb0ed734f3ccc059b7b77e3 didn't
do what it purported, because it omitted parentheses that
were necessary to change the natural precedence.  It's strange that
it passed all tests on my machine, and failed so miserably elsewhere
that it was quickly reverted by commit
63c0bfd59562325fed2ac5a90088ed40960ac2ad.

This reinstates it with the correct precedence.  The next commit
will add an assert() so that the underlying issue will be detected
on all platforms