match is going to fail, PCRE2 has in principle to try every possible
variation, and this can take an extremely long time, even for relatively short
strings.
</P>
<P>
An optimization catches some of the more simple cases such as
<pre>
  (a+)*b
</pre>
where a literal character follows. Before embarking on the standard matching
procedure, PCRE2 checks that there is a "b" later in the subject string, and if
there is not, it fails the match immediately. However, when there is no
following literal this optimizati