RegExp.leftContext ($`)
Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.
Note:
All RegExp
static properties that expose the last match state globally are deprecated. See deprecated RegExp features for more information.
The RegExp.leftContext
static accessor property returns the substring preceding the most recent match. RegExp["$`"]
is an alias for this property.
Description
Because leftContext
is a static property of RegExp
, you always use it as RegExp.leftContext
or RegExp["$`"]
, rather than as a property of a RegExp
object you created.
The value of leftContext
updates whenever a RegExp
(but not a RegExp
subclass) instance makes a successful match. If no matches have been made, leftContext
is an empty string. The set accessor of leftContext
is undefined
, so you cannot change this property directly.
You cannot use the shorthand alias with the dot property accessor (RegExp.$`
), because `
is not a valid identifier part, so this causes a SyntaxError
. Use the bracket notation instead.
$`
can also be used in the replacement string of String.prototype.replace()
, but that's unrelated to the RegExp["$`"]
legacy property.
Examples
Using leftContext and $`
const re = /world/g;
re.test("hello world!");
RegExp.leftContext; // "hello "
RegExp["$`"]; // "hello "
Specifications
Specification |
---|
Legacy RegExp features # additional-properties-of-the-regexp-constructor |
Browser compatibility
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