updated for version 7.3.1093
Problem: New regexp engine: When a sub expression is empty \1 skips a
character.
Solution: Make \1 try the current position when the match is emtpy.
diff --git a/src/testdir/test64.in b/src/testdir/test64.in
index 7890924..4d892af 100644
--- a/src/testdir/test64.in
+++ b/src/testdir/test64.in
@@ -361,6 +361,7 @@
:call add(tl, [2, '\(\i\+\) \1', ' abc abc', 'abc abc', 'abc'])
:call add(tl, [2, '\(\i\+\) \1', 'xgoo goox', 'goo goo', 'goo'])
:call add(tl, [2, '\(a\)\(b\)\(c\)\(dd\)\(e\)\(f\)\(g\)\(h\)\(i\)\1\2\3\4\5\6\7\8\9', 'xabcddefghiabcddefghix', 'abcddefghiabcddefghi', 'a', 'b', 'c', 'dd', 'e', 'f', 'g', 'h', 'i'])
+:call add(tl, [2, '\(\d*\)a \1b', ' a b ', 'a b', ''])
:"
:"""" Look-behind with limit
:call add(tl, [2, '<\@<=span.', 'xxspanxx<spanyyy', 'spany'])