vim-patch:8.2.4978: no error if engine selection atom is not at the start

Problem:    No error if engine selection atom is not at the start.
Solution:   Give an error. (Christian Brabandt, closes vim/vim#10439)

360da40b47

Co-authored-by: Christian Brabandt <cb@256bit.org>

runtime/doc/pattern.txt

@@ -372,7 +372,7 @@ Vim includes two regexp engines:
 1. An old, backtracking engine that supports everything.
 2. A new, NFA engine that works much faster on some patterns, possibly slower
    on some patterns.
-
+								 *E1281*
 Vim will automatically select the right engine for you.  However, if you run
 into a problem or want to specifically select one engine or the other, you can
 prepend one of the following to the pattern:

src/nvim/regexp.c

@@ -112,6 +112,8 @@ static char e_regexp_number_after_dot_pos_search_chr[]
   = N_("E1204: No Number allowed after .: '\\%%%c'");
 static char e_nfa_regexp_missing_value_in_chr[]
   = N_("E1273: (NFA regexp) missing value in '\\%%%c'");
+static char e_atom_engine_must_be_at_start_of_pattern[]
+  = N_("E1281: Atom '\\%%#=%c' must be at the start of the pattern");
 static char e_substitute_nesting_too_deep[] = N_("E1290: substitute nesting too deep");
 
 #define NOT_MULTI       0

src/nvim/regexp_bt.c

@@ -1971,6 +1971,11 @@ static char_u *regatom(int *flagp)
       break;
 
     case '#':
+      if (regparse[0] == '=' && regparse[1] >= 48 && regparse[1] <= 50) {
+        // misplaced \%#=1
+        semsg(_(e_atom_engine_must_be_at_start_of_pattern), regparse[1]);
+        return FAIL;
+      }
       ret = regnode(CURSOR);
       break;
 

src/nvim/regexp_nfa.c

@@ -2094,6 +2094,12 @@ static int nfa_regatom(void)
       break;
 
     case '#':
+      if (regparse[0] == '=' && regparse[1] >= 48
+          && regparse[1] <= 50) {
+        // misplaced \%#=1
+        semsg(_(e_atom_engine_must_be_at_start_of_pattern), regparse[1]);
+        return FAIL;
+      }
       EMIT(NFA_CURSOR);
       break;
 

src/nvim/testdir/test_regexp_latin.vim

@@ -1058,6 +1058,24 @@ func Test_using_invalid_visual_position()
   bwipe!
 endfunc
 
+func Test_using_two_engines_pattern()
+  new
+  call setline(1, ['foobar=0', 'foobar=1', 'foobar=2'])
+  " \%#= at the end of the pattern
+  for i in range(0, 2)
+    call cursor( (i+1), 7) 
+    call assert_fails("%s/foobar\\%#=" .. i, 'E1281:')
+  endfor
+
+  " \%#= at the start of the pattern
+  for i in range(0, 2)
+    call cursor( (i+1), 7) 
+    exe ":%s/\\%#=" .. i .. "foobar=" .. i .. "/xx"
+  endfor
+  call assert_equal(['xx', 'xx', 'xx'], getline(1, '$'))
+  bwipe!
+endfunc
+
 func Test_recursive_substitute_expr()
   new
   func Repl()