Prevent backreferences from breaking regex.transform().

src/util/regex.py

@@ -38,7 +38,16 @@ def replace(s, pattern, replacement):
         pattern (str):     the search expression
         replacement (str): the string to replace each match with
     """
-    return re.sub(pattern, replacement, s)
+    # the replacement string may contain invalid backreferences (like \1 or \g)
+    # which will cause python's regex to blow up. Since this should emulate
+    # the jam version exactly and the jam version didn't support
+    # backreferences, this version shouldn't either. re.sub
+    # allows replacement to be a callable; this is being used
+    # to simply return the replacement string and avoid the hassle
+    # of worrying about backreferences within the string.
+    def _replacement(matchobj):
+        return replacement
+    return re.sub(pattern, _replacement, s)
 
 
 @bjam_signature((['items', '*'], ['match'], ['replacement']))