RegExp.leftContext ($`) - JavaScript 编辑

Non-standard

This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.

The non-standard leftContext property is a static and read-only property of regular expressions that contains the substring preceding the most recent match. RegExp.$` is an alias for this property.

Description

The leftContext property is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.leftContext or RegExp['$`'].

The value of the leftContext property is read-only and modified whenever a successful match is made.

You can not use the shorthand alias with the dot property accessor (RegExp.$`), because the parser expects a starting template string in that case and a SyntaxError is thrown. Use the bracket notation for property access.

Examples

Using leftContext and $`

var re = /world/g;
re.test('hello world!');
RegExp.leftContext; // "hello "
RegExp['$`'];       // "hello "

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• RegExp.input ($_)
• RegExp.lastMatch ($&)
• RegExp.lastParen ($+)
• RegExp.rightContext ($')
• RegExp.$1-$9