Describe what happens to $'s that don't fit the rule.

pep-0292.txt

@@ -61,6 +61,11 @@ A Simpler Proposal
        when valid identifier characters follow the placeholder but are
        not part of the placeholder, e.g. "${noun}ification".
 
+    If the $ character appears at the end of the line, or is followed
+    by any other character than those described above, it is treated
+    as if it had been escaped, appearing in the resulting string
+    unchanged.
+
     No other characters have special meaning, however it is possible
     to derive from the dstring class to define different rules for the
     placeholder.  For example, a derived class could allow for periods