--- title: RegExp.leftContext ($`) slug: Web/JavaScript/Reference/Global_Objects/RegExp/leftContext translation_of: Web/JavaScript/Reference/Global_Objects/RegExp/leftContext --- <div>{{JSRef}} {{non-standard_header}}</div> <p><strong>leftContext </strong>非标准属性是正则表达式的静态和只读属性,含有最新匹配的左侧子串。 <code>RegExp.$`</code> 是这个属性的别名。</p> <h2 id="语法">语法</h2> <pre class="syntaxbox"><var>RegExp</var>.leftContext RegExp['$`'] </pre> <h2 id="描述">描述</h2> <p><code>leftContext</code> 属性是静态的,不是正则表达式独立对象的属性。反之,你应始终将其使用为 <code>RegExp.leftContext</code> 或者 <code>RegExp['$`']</code>。</p> <p><code>leftContext</code> 属性的值是只读的,并且会在匹配成功时修改。</p> <p>你不能使用属性访问器(<code>RegExp.$`</code>)来使用简写的别名,因为解析器在这里会将其看做模板字符串的开始,并抛出 {{jsxref("SyntaxError")}} 。使用 <a href="/en-US/docs/Web/JavaScript/Reference/Operators/Property_Accessors">方括号符号</a>来访问属性。</p> <h2 id="示例">示例</h2> <h3 id="使用_leftContext_和">使用 <code>leftContext</code> 和 <code>$`</code></h3> <pre class="brush: js">var re = /world/g; re.test('hello world!'); RegExp.leftContext; // "hello " RegExp['$`']; // "hello " </pre> <h2 id="规范">规范</h2> <p>非标准。并不是任何现行规范的一部分。</p> <h2 id="浏览器兼容性">浏览器兼容性</h2> <div>{{CompatibilityTable}}</div> <div id="compat-desktop"> <table class="compat-table"> <tbody> <tr> <th>Feature</th> <th>Chrome</th> <th>Firefox (Gecko)</th> <th>Internet Explorer</th> <th>Opera</th> <th>Safari</th> </tr> <tr> <td>Basic support</td> <td>{{CompatVersionUnknown}}</td> <td>{{CompatVersionUnknown}}</td> <td>{{CompatVersionUnknown}}</td> <td>{{CompatVersionUnknown}}</td> <td>{{CompatVersionUnknown}}</td> </tr> </tbody> </table> </div> <div id="compat-mobile"> <table class="compat-table"> <tbody> <tr> <th>Feature</th> <th>Android</th> <th>Chrome for Android</th> <th>Firefox Mobile (Gecko)</th> <th>IE Mobile</th> <th>Opera Mobile</th> <th>Safari Mobile</th> </tr> <tr> <td>Basic support</td> <td>{{CompatVersionUnknown}}</td> <td>{{CompatVersionUnknown}}</td> <td>{{CompatVersionUnknown}}</td> <td>{{CompatVersionUnknown}}</td> <td>{{CompatVersionUnknown}}</td> <td>{{CompatVersionUnknown}}</td> </tr> </tbody> </table> </div> <h2 id="另见">另见</h2> <ul> <li>{{non-standard_inline}} {{jsxref("RegExp.input", "RegExp.input ($_)")}}</li> <li>{{non-standard_inline}} {{jsxref("RegExp.lastMatch", "RegExp.lastMatch ($&)")}}</li> <li>{{non-standard_inline}} {{jsxref("RegExp.lastParen", "RegExp.lastParen ($+)")}}</li> <li>{{non-standard_inline}} {{jsxref("RegExp.rightContext", "RegExp.rightContext ($')")}}</li> <li>{{non-standard_inline}} {{jsxref("RegExp.n", "RegExp.$1-$9")}}</li> </ul>