properly support lookahead expresions in matches

- passes the FULL remaining buffer to `processLexeme` so
  that when it attemps to detemine the next mode it can
  run the ruleset against the FULL buffer, not just the
  terminator which resulted in a match (which will not
  be sufficient enough since lookaheads aren't included
  in the actual match)

src/highlight.js

@@ -487,7 +487,7 @@ https://highlightjs.org/
       top = Object.create(mode, {parent: {value: top}});
     }
 
-    function processLexeme(buffer, lexeme) {
+    function processLexeme(buffer, lexeme, remaining_buffer) {
 
       mode_buffer += buffer;
 
@@ -496,7 +496,11 @@ https://highlightjs.org/
         return 0;
       }
 
-      var new_mode = subMode(lexeme, top);
+      // we use remaining_buffer here rather than lexeme so that
+      // positive/negative lookaheads can work, even though the
+      // lookahead content would not be included in the lexeme itself
+      // (as it's technically NOT part of the match)
+      var new_mode = subMode(remaining_buffer, top);
       if (new_mode) {
         if (new_mode.skip) {
           mode_buffer += lexeme;
@@ -573,14 +577,18 @@ https://highlightjs.org/
     }
     var mode_buffer = '';
     var relevance = 0;
+    var remaining_buffer, skipped_buffer;
     try {
       var match, count, index = 0;
       while (true) {
         top.terminators.lastIndex = index;
         match = top.terminators.exec(value);
         if (!match)
           break;
-        count = processLexeme(value.substring(index, match.index), match[0]);
+        // buffer between the last match and the current match
+        skipped_buffer = value.substring(index, match.index)
+        remaining_buffer = value.substr(match.index)
+        count = processLexeme(skipped_buffer, match[0], remaining_buffer);
         index = match.index + count;
       }
       processLexeme(value.substr(index));