the time when used on a similar string with 1000000 C<a>s. Be aware,
however, that this pattern currently triggers a warning message under
the C<use warnings> pragma or B<-w> switch saying it
-C<"matches the null string many times">):
+C<"matches null string many times in regex">.
On simple groups, such as the pattern C<< (?> [^()]+ ) >>, a comparable
effect may be achieved by negative look-ahead, as in C<[^()]+ (?! [^()] )>.