Index: Lib/sre_parse.py
===================================================================
--- Lib/sre_parse.py    (revision 74321)
+++ Lib/sre_parse.py    (working copy)
@@ -583,7 +583,8 @@
                     dir = 1
                     if char == "<":
                         if source.next not in LOOKBEHINDASSERTCHARS:
-                            raise error, "syntax error"
+                            raise error, ("bad lookbehind assertion type: %s" %
+                                    source.next)
                         dir = -1 # lookbehind
                         char = sourceget()
                     p = _parse_sub(source, state)