Core: Fixed greedy partial lookbehinds not working (#2030)

This fixes the bug that greedy patterns with a lookbehind group did not support other top-level alternatives (e.g. `/foo|(lookbehind)bar/`) causing an error when used.

components/prism-core.js

@@ -327,7 +327,7 @@ var _ = {
 							break;
 						}
 
-						var from = match.index + (lookbehind ? match[1].length : 0),
+						var from = match.index + (lookbehind && match[1] ? match[1].length : 0),
 						    to = match.index + match[0].length,
 						    k = i,
 						    p = pos;

prism.js

@@ -332,7 +332,7 @@ var _ = {
 							break;
 						}
 
-						var from = match.index + (lookbehind ? match[1].length : 0),
+						var from = match.index + (lookbehind && match[1] ? match[1].length : 0),
 						    to = match.index + match[0].length,
 						    k = i,
 						    p = pos;

tests/core/greedy.js

@@ -35,6 +35,26 @@ describe('Greedy matching', function () {
 		});
 	});
 
+	it('should support patterns with top-level alternatives that do not contain the lookbehind group', function () {
+		testTokens({
+			grammar: {
+				'a': /'[^']*'/,
+				'b': {
+					// This pattern has 2 top-level alternatives:  foo  and  (^|[^\\])"[^"]*"
+					pattern: /foo|(^|[^\\])"[^"]*"/,
+					lookbehind: true,
+					greedy: true
+				}
+			},
+			code: 'foo "bar" \'baz\'',
+			expected: [
+				["b", "foo"],
+				["b", "\"bar\""],
+				["a", "'baz'"]
+			]
+		});
+	});
+
 	// https://github.com/PrismJS/prism/issues/1492
 	/*
 	it('should correctly rematch tokens', function () {
@@ -66,4 +86,3 @@ describe('Greedy matching', function () {
 	});
 	*/
 });
-