From cfb2383c26847e9332d1283fa15753b09d322f2f Mon Sep 17 00:00:00 2001
From: Jan Edmund Lazo <janedmundlazo@hotmail.com>
Date: Sun, 12 Aug 2018 18:06:17 -0400
Subject: [PATCH] vim-patch:8.0.1397: pattern with \& following nothing gives
 an error

Problem:    Pattern with \& following nothing gives an error.
Solution:   Emit an empty node when needed.
https://github.com/vim/vim/commit/890dd05492d88d48eee1dda7f7a1811d027ce7ca
---
 src/nvim/regexp_nfa.c            | 9 +++++----
 src/nvim/testdir/test_search.vim | 8 ++++++++
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/src/nvim/regexp_nfa.c b/src/nvim/regexp_nfa.c
index 3b905f5efc..2536de27c1 100644
--- a/src/nvim/regexp_nfa.c
+++ b/src/nvim/regexp_nfa.c
@@ -2131,7 +2131,6 @@ static int nfa_regconcat(void)
  */
 static int nfa_regbranch(void)
 {
-  int ch;
   int old_post_pos;
 
   old_post_pos = (int)(post_ptr - post_start);
@@ -2140,10 +2139,13 @@ static int nfa_regbranch(void)
   if (nfa_regconcat() == FAIL)
     return FAIL;
 
-  ch = peekchr();
   /* Try next concats */
-  while (ch == Magic('&')) {
+  while (peekchr() == Magic('&')) {
     skipchr();
+    // if concat is empty do emit a node
+    if (old_post_pos == (int)(post_ptr - post_start)) {
+      EMIT(NFA_EMPTY);
+    }
     EMIT(NFA_NOPEN);
     EMIT(NFA_PREV_ATOM_NO_WIDTH);
     old_post_pos = (int)(post_ptr - post_start);
@@ -2153,7 +2155,6 @@ static int nfa_regbranch(void)
     if (old_post_pos == (int)(post_ptr - post_start))
       EMIT(NFA_EMPTY);
     EMIT(NFA_CONCAT);
-    ch = peekchr();
   }
 
   /* if a branch is empty, emit one node for it */
diff --git a/src/nvim/testdir/test_search.vim b/src/nvim/testdir/test_search.vim
index 42c56dced7..7663c9e283 100644
--- a/src/nvim/testdir/test_search.vim
+++ b/src/nvim/testdir/test_search.vim
@@ -472,3 +472,11 @@ endfunc
 func Test_search_undefined_behaviour2()
   call search("\%UC0000000")
 endfunc
+
+" This was causing E874.  Also causes an invalid read?
+func Test_look_behind()
+  new
+  call setline(1, '0\|\&\n\@<=') 
+  call search(getline("."))
+  bwipe!
+endfunc