:p is incompatible with scanners

:p may not be used with :ex, :ov, :g, :nth, or :x modifiers.
fixes #16

S05-regex.pod

@@ -17,8 +17,8 @@ Synopsis 5: Regexes and Rules
 
     Created: 24 Jun 2002
 
-    Last Modified: 23 May 2012
-    Version: 156
+    Last Modified: 28 Jul 2012
+    Version: 157
 
 This document summarizes Apocalypse 5, which is about the new regex
 syntax.  We now try to call them I<regex> rather than "regular
@@ -314,6 +314,10 @@ is roughly equivalent to
 
      m:p($p)/.*? <( pattern )> /
 
+All of C<:g>, C<:ov>, C<:nth>, and C<:x> are incompatible with C<:p> and
+will fail, recommending use of C<:c> instead.  The C<:ex> modifier is allowed
+but will produce only matches at that position.
+
 =item *
 
 The new C<:s> (C<:sigspace>) modifier causes certain whitespace sequences