+=head2 free
+
+ use re::engine::Plugin(
+ free => sub {
+ my ($rx) = @_;
+
+ say 'matched ' ($ponies // 'no')
+ . ' pon' . ($ponies > 1 ? 'ies' : 'y');
+
+ return;
+ }
+ );
+
+Called when the regexp structure is freed by the perl interpreter.
+Note that this happens pretty late in the destruction process, but
+still before global destruction kicks in. The only argument this
+callback receives is the C<re::engine::Plugin> object associated
+with the regexp, and its return value is ignored.
+
+This callback can also be specified on an individual basis with the
+L</callbacks> method.
+