In RegExp, lastIndex is read with ToLength, not ToInteger

src/regexp.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 var $regexpLastMatchInfoOverride;
 var harmony_regexps = false;
 var harmony_unicode_regexps = false;
 
 (function(global, utils) {
 
@@ skipping 136 matching lines @@
 
 function RegExpExecJS(string) {
   if (!IS_REGEXP(this)) {
     throw MakeTypeError(kIncompatibleMethodReceiver,
                         'RegExp.prototype.exec', this);
   }
 
   string = TO_STRING_INLINE(string);
   var lastIndex = this.lastIndex;
 
-  // Conversion is required by the ES5 specification (RegExp.prototype.exec
-  // algorithm, step 5) even if the value is discarded for non-global RegExps.
-  var i = TO_INTEGER(lastIndex);
+  // Conversion is required by the ES6 specification (RegExpBuiltinExec
+  // algorithm, step 4) even if the value is discarded for non-global RegExps.
+  var i = $toLength(lastIndex);
 
   var updateLastIndex = this.global || (harmony_regexps && this.sticky);
   if (updateLastIndex) {
     if (i < 0 || i > string.length) {
       this.lastIndex = 0;
       return null;
     }
   } else {
     i = 0;
   }
@@ skipping 25 matching lines @@
 // else implements.
 function RegExpTest(string) {
   if (!IS_REGEXP(this)) {
     throw MakeTypeError(kIncompatibleMethodReceiver,
                         'RegExp.prototype.test', this);
   }
   string = TO_STRING_INLINE(string);
 
   var lastIndex = this.lastIndex;
 
-  // Conversion is required by the ES5 specification (RegExp.prototype.exec
-  // algorithm, step 5) even if the value is discarded for non-global RegExps.
-  var i = TO_INTEGER(lastIndex);
+  // Conversion is required by the ES6 specification (RegExpBuiltinExec
+  // algorithm, step 4) even if the value is discarded for non-global RegExps.
+  var i = $toLength(lastIndex);
 
   if (this.global || (harmony_regexps && this.sticky)) {
     if (i < 0 || i > string.length) {
       this.lastIndex = 0;
       return false;
     }
     // matchIndices is either null or the RegExpLastMatchInfo array.
     var matchIndices = %_RegExpExec(this, string, i, RegExpLastMatchInfo);
     if (IS_NULL(matchIndices)) {
       this.lastIndex = 0;
@@ skipping 226 matching lines @@
 // Exports
 
 utils.Export(function(to) {
   to.RegExpExec = DoRegExpExec;
   to.RegExpExecNoTests = RegExpExecNoTests;
   to.RegExpLastMatchInfo = RegExpLastMatchInfo;
   to.RegExpTest = RegExpTest;
 });
 
 })