If @var{laxmatch} is non-@code{nil}, it means there should be no error
if there is no subexpression numbered @var{subexp} in @var{matcher}.
+Obviously, fontification of the subexpression numbered @var{subexp} will
+not occur. However, fontification of other subexpressions (and other
+regexps) will continue. If @var{laxmatch} is @code{nil}, and the
+specified subexpression is missing, then an error is signalled which
+terminates search-based fontification.
Here are some examples of elements of this kind, and what they do: