Add rough note about lookbehind assertions

I'm sure this example can be improved upon.  This is a start nevertheless...

Author: paultcochrane

lib/Language/regexes.pod

@@ -904,6 +904,37 @@ abracadabra
        abra
 =end code
 
+=head1 Look-around assertions
+
+=head2 Lookahead assertions
+
+=head2 Lookbehind assertions
+
+To check that a pattern appears before another pattern, one can use a
+lookbehind assertion via the C<after> assertion.  This has the form:
+
+    <?after pattern>
+
+Thus, to search for the string C<bar> which is immediately preceded by the
+string C<foo>, one could use the following regexp:
+
+    rx{ <?after foo> bar }
+
+which one could use like so:
+
+    say "foobar" ~~ rx{ <?after foo> bar };   #->  bar
+
+However, if you want to search for a pattern which is B<not> immediately
+preceded by some pattern, then you need to use a negative lookbehind
+assertion, this has the form:
+
+    <?!after pattern>
+
+Hence all occurrences of C<bar> which I<do not> have C<foo> before them
+would be matched by
+
+    rx{ <?!after foo> bar }
+
 =head1 Best practices and gotchas
 
 Regexes and L<grammars|/language/grammars> are a whole programming paradigm