Fix: Don't allow regex y flag in ES5

espree.js

@@ -209,6 +209,14 @@ function isValidToken(parser) {
         case tt.jsxTagEnd:
             return ecma.jsx;
 
+        // https://github.com/ternjs/acorn/issues/363
+        case tt.regexp:
+            if (extra.ecmaVersion < 6 && parser.value.flags && parser.value.flags.indexOf("y") > -1) {
+                return false;
+            }
+
+            return true;
+
         default:
             return true;
     }
@@ -427,7 +435,7 @@ function tokenize(code, options) {
     options = options || {};
 
     var acornOptions = {
-        ecmaVersion: 6
+        ecmaVersion: 5
     };
 
     resetExtra();
@@ -453,6 +461,20 @@ function tokenize(code, options) {
 
     extra.tolerant = typeof options.tolerant === "boolean" && options.tolerant;
 
+    if (typeof options.ecmaVersion === "number") {
+        switch (options.ecmaVersion) {
+            case 3:
+            case 5:
+            case 6:
+                acornOptions.ecmaVersion = options.ecmaVersion;
+                extra.ecmaVersion = options.ecmaVersion;
+                break;
+
+            default:
+                throw new Error("ecmaVersion must be 3, 5, or 6.");
+        }
+    }
+
     // apply parsing flags
     if (options.ecmaFeatures && typeof options.ecmaFeatures === "object") {
         extra.ecmaFeatures = options.ecmaFeatures;

tests/lib/parse.js

@@ -53,6 +53,24 @@ describe("parse()", function() {
 
     });
 
+    describe("ES5", function() {
+
+        it("should throw an error when using the y regex flag", function() {
+
+            assert.throws(function() {
+                espree.parse("/./y");
+            });
+        });
+
+        it("should throw an error when using the u regex flag", function() {
+
+            assert.throws(function() {
+                espree.parse("/./u");
+            });
+        });
+
+    });
+
     describe("general", function() {
         it("should output tokens, comments, locs, and ranges when called with those options", function() {
             var ast = espree.parse("let foo = bar;", {