+=head2 nparens
+
+=head2 gofs
+
+=head2 callbacks
+
+ # A dumb regexp engine that just tests string equality
+ use re::engine::Plugin comp => sub {
+ my ($re) = @_;
+
+ my $pat = $re->pattern;
+
+ $re->callbacks(
+ exec => sub {
+ my ($re, $str) = @_;
+ return $pat eq $str;
+ },
+ );
+ };
+
+Takes a list of key-value pairs of names and subroutines, and replace the
+callback currently attached to the regular expression for the type given as
+the key by the code reference passed as the corresponding value.
+
+The only valid key is currently C<exec>. See L</exec> for more details about
+this callback.
+