Revision 984283 of regexp.lastIndex

<div>{{JSRef}}</div>

<p>The <strong><code>lastIndex</code></strong> is a read/write integer property of regular expressions that specifies the index at which to start the next match.</p>

<div>{{js_property_attributes(1, 0, 0)}}</div>

<h2 id="Syntax">Syntax</h2>

<pre class="syntaxbox">
<code><var>regExpObj</var>.lastIndex</code></pre>

<h2 id="Description">Description</h2>

<p>This property is set only if the regular expression used the <code>"g"</code> flag to indicate a global search. The following rules apply:</p>

<ul>
 <li>If <code>lastIndex</code> is greater than the length of the string, {{jsxref("RegExp.prototype.test()", "test()")}} and {{jsxref("RegExp.prototype.exec()", "exec()")}} fail, then <code>lastIndex</code> is set to 0.</li>
 <li>If <code>lastIndex</code> is equal to the length of the string and if the regular expression matches the empty string, then the regular expression matches input starting at <code>lastIndex</code>.</li>
 <li>If <code>lastIndex</code> is equal to the length of the string and if the regular expression does not match the empty string, then the regular expression mismatches input, and <code>lastIndex</code> is reset to 0.</li>
 <li>Otherwise, <code>lastIndex</code> is set to the next position following the most recent match.</li>
</ul>

<h2 id="Examples">Examples</h2>

<h3 id="Using_lastIndex">Using <code>lastIndex</code></h3>

<p>Consider the following sequence of statements:</p>

<pre class="brush: js">
var re = /(hi)?/g;
</pre>

<p>Matches the empty string.</p>

<pre class="brush: js">
console.log(re.exec('hi'));
console.log(re.lastIndex);
</pre>

<p>Returns <code>["hi", "hi"]</code> with <code>lastIndex</code> equal to 2.</p>

<pre class="brush: js">
console.log(re.exec('hi'));
console.log(re.lastIndex);
</pre>

<p>Returns <code>["", undefined]</code>, an empty array whose zeroth element is the match string. In this case, the empty string because <code>lastIndex</code> was 2 (and still is 2) and <code>"hi"</code> has length 2.</p>

<h2 id="Specifications">Specifications</h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
  <tr>
   <td>{{SpecName('ES3')}}</td>
   <td>{{Spec2('ES3')}}</td>
   <td>Initial definition. Implemented in JavaScript 1.2. JavaScript 1.5: <code>lastIndex</code> is a property of a {{jsxref("RegExp")}} instance, not the {{jsxref("RegExp")}} object.</td>
  </tr>
  <tr>
   <td>{{SpecName('ES5.1', '#sec-15.10.7.5', 'RegExp.lastIndex')}}</td>
   <td>{{Spec2('ES5.1')}}</td>
   <td>&nbsp;</td>
  </tr>
  <tr>
   <td>{{SpecName('ES6', '#sec-properties-of-regexp-instances', 'RegExp.lastIndex')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>&nbsp;</td>
  </tr>
  <tr>
   <td>{{SpecName('ESDraft', '#sec-properties-of-regexp-instances', 'RegExp.lastIndex')}}</td>
   <td>{{Spec2('ESDraft')}}</td>
   <td>&nbsp;</td>
  </tr>
 </tbody>
</table>

<h2 id="Browser_compatibility">Browser compatibility</h2>

<div>{{CompatibilityTable}}</div>

<div id="compat-desktop">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Chrome</th>
   <th>Firefox (Gecko)</th>
   <th>Internet Explorer</th>
   <th>Opera</th>
   <th>Safari</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
  </tr>
 </tbody>
</table>
</div>

<div id="compat-mobile">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Android</th>
   <th>Chrome for Android</th>
   <th>Firefox Mobile (Gecko)</th>
   <th>IE Mobile</th>
   <th>Opera Mobile</th>
   <th>Safari Mobile</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatVersionUnknown}}</td>
  </tr>
 </tbody>
</table>
</div>

<h2 id="See_also">See also</h2>

<ul>
 <li>{{jsxref("RegExp.prototype.ignoreCase")}}</li>
 <li>{{jsxref("RegExp.prototype.global")}}</li>
 <li>{{jsxref("RegExp.prototype.multiline")}}</li>
 <li>{{jsxref("RegExp.prototype.source")}}</li>
 <li>{{jsxref("RegExp.prototype.sticky")}}</li>
</ul>