Improved error msg when a symbolic group name is redefined. Added docs

and NEWS.  Bugfix candidate?  That's a dilemma for Anthony <wink>:  /F
did fix a longstanding bug here, but the fix can cause code to raise an
exception that previously worked by accident.

Doc/lib/libre.tex

@@ -237,7 +237,8 @@ referenced later in the pattern.
 
 \item[\code{(?P<\var{name}>...)}] Similar to regular parentheses, but
 the substring matched by the group is accessible via the symbolic group
-name \var{name}.  Group names must be valid Python identifiers.  A
+name \var{name}.  Group names must be valid Python identifiers, and
+each group name must be defined only once within a regular expression.  A
 symbolic group is also a numbered group, just as if the group were not
 named.  So the group named 'id' in the example above can also be
 referenced as the numbered group 1.

Lib/sre_parse.py

@@ -81,8 +81,10 @@ class Pattern:
         gid = self.groups
         self.groups = gid + 1
         if name:
-            if self.groupdict.has_key(name):
-                raise error, "can only use each group name once"
+            ogid = self.groupdict.get(name, None)
+            if ogid is not None:
+                raise error, ("redefinition of group name %s as group %d; " +
+                              "was group %d") % (`name`, gid,  ogid)
             self.groupdict[name] = gid
         self.open.append(gid)
         return gid

Misc/NEWS

@@ -46,6 +46,12 @@ Extension modules
 
 Library
 
+- Symbolic group names in regular expressions must be unique.  For
+  example, the regexp r'(?P<abc>)(?P<abc>)' is not allowed, because a
+  single name can't mean both "group 1" and "group 2" simultaneously.
+  Python 2.2 detects this error at regexp compilation time; previously,
+  the error went undetected, and results were unpredictable.
+
 - Tix exposes more commands through the classes DirSelectBox,
   DirSelectDialog, ListNoteBook, Meter, CheckList, and the
   methods tix_addbitmapdir, tix_cget, tix_configure, tix_filedialog,