String.prototype.replace()

The replace() method returns a new string with some or all matches of a pattern replaced by a replacement. The pattern can be a string or a RegExp, and the replacement can be a string or a function to be called for each match. If pattern is a string, only the first occurrence will be replaced.

The original string is left unchanged.

Syntax

const newStr = str.replace(regexp|substr, newSubstr|function)

Parameters

regexp (pattern)
A RegExp object or literal. The match or matches are replaced with newSubstr or the value returned by the specified function.
substr
A String that is to be replaced by newSubstr. It is treated as a literal string and is not interpreted as a regular expression. Only the first occurrence will be replaced.
newSubstr (replacement)
The String that replaces the substring specified by the specified regexp or substr parameter. A number of special replacement patterns are supported; see the "Specifying a string as a parameter" section below.
function (replacement)
A function to be invoked to create the new substring to be used to replace the matches to the given regexp or substr. The arguments supplied to this function are described in the "Specifying a function as a parameter" section below.

Return value

A new string, with some or all matches of a pattern replaced by a replacement.

Description

This method does not change the calling String object. It simply returns a new string.

To perform a global search and replace, include the g switch in the regular expression.

Specifying a string as a parameter

The replacement string can include the following special replacement patterns:

Pattern Inserts
$$ Inserts a "$".
$& Inserts the matched substring.
$` Inserts the portion of the string that precedes the matched substring.
$' Inserts the portion of the string that follows the matched substring.
$n Where n is a positive integer less than 100, inserts the nth parenthesized submatch string, provided the first argument was a RegExp object. Note that this is 1-indexed.

Specifying a function as a parameter

You can specify a function as the second parameter. In this case, the function will be invoked after the match has been performed. The function's result (return value) will be used as the replacement string. (Note: The above-mentioned special replacement patterns do not apply in this case.)

Note that the function will be invoked multiple times for each full match to be replaced if the regular expression in the first parameter is global.

The arguments to the function are as follows:

Possible name Supplied value
match The matched substring. (Corresponds to $& above.)
p1, p2, ... The nth string found by a parenthesized capture group, provided the first argument to replace() was a RegExp object. (Corresponds to $1, $2, etc. above.) For example, if /(\a+)(\b+)/, was given, p1 is the match for \a+, and p2 for \b+.
offset The offset of the matched substring within the whole string being examined. (For example, if the whole string was 'abcd', and the matched substring was 'bc', then this argument will be 1.)
string The whole string being examined.

(The exact number of arguments depends on whether the first argument is a RegExp object—and, if so, how many parenthesized submatches it specifies.)

The following example will set newString to 'abc - 12345 - #$*%':

function replacer(match, p1, p2, p3, offset, string) {
  // p1 is nondigits, p2 digits, and p3 non-alphanumerics
  return [p1, p2, p3].join(' - ');
}
let newString = 'abc12345#$*%'.replace(/([^\d]*)(\d*)([^\w]*)/, replacer);
console.log(newString);  // abc - 12345 - #$*%

Examples

Defining the regular expression in replace()

In the following example, the regular expression is defined in replace() and includes the ignore case flag.

let str = 'Twas the night before Xmas...';
let newstr = str.replace(/xmas/i, 'Christmas');
console.log(newstr);  // Twas the night before Christmas...

This logs 'Twas the night before Christmas...'.

Note: See this guide for more explanations about regular expressions.

Using global and ignore with replace()

Global replace can only be done with a regular expression. In the following example, the regular expression includes the global and ignore case flags which permits replace() to replace each occurrence of 'apples' in the string with 'oranges'.

let re = /apples/gi;
let str = 'Apples are round, and apples are juicy.';
let newstr = str.replace(re, 'oranges');
console.log(newstr);  // oranges are round, and oranges are juicy.

This logs 'oranges are round, and oranges are juicy'.

Switching words in a string

The following script switches the words in the string. For the replacement text, the script uses capturing groups and the $1 and $2 replacement patterns.

let re = /(\w+)\s(\w+)/;
let str = 'John Smith';
let newstr = str.replace(re, '$2, $1');
console.log(newstr);  // Smith, John

This logs 'Smith, John'.

Using an inline function that modifies the matched characters

In this example, all occurrences of capital letters in the string are converted to lower case, and a hyphen is inserted just before the match location. The important thing here is that additional operations are needed on the matched item before it is given back as a replacement.

The replacement function accepts the matched snippet as its parameter, and uses it to transform the case and concatenate the hyphen before returning.

function styleHyphenFormat(propertyName) {
  function upperToHyphenLower(match, offset, string) {
    return (offset > 0 ? '-' : '') + match.toLowerCase();
  }
  return propertyName.replace(/[A-Z]/g, upperToHyphenLower);
}

Given styleHyphenFormat('borderTop'), this returns 'border-top'.

Because we want to further transform the result of the match before the final substitution is made, we must use a function. This forces the evaluation of the match prior to the toLowerCase() method. If we had tried to do this using the match without a function, the toLowerCase() would have no effect.

let newString = propertyName.replace(/[A-Z]/g, '-' + '$&'.toLowerCase());  // won't work

This is because '$&'.toLowerCase() would first be evaluated as a string literal (resulting in the same '$&') before using the characters as a pattern.

Replacing a Fahrenheit degree with its Celsius equivalent

The following example replaces a Fahrenheit degree with its equivalent Celsius degree. The Fahrenheit degree should be a number ending with "F". The function returns the Celsius number ending with "C". For example, if the input number is "212F", the function returns "100C". If the number is "0F", the function returns "-17.77777777777778C".

The regular expression test checks for any number that ends with F. The number of Fahrenheit degree is accessible to the function through its second parameter, p1. The function sets the Celsius number based on the Fahrenheit degree passed in a string to the f2c() function. f2c() then returns the Celsius number. This function approximates Perl's s///e flag.

function f2c(x) {
  function convert(str, p1, offset, s) {
    return ((p1 - 32) * 5/9) + 'C';
  }
  let s = String(x);
  let test = /(-?\d+(?:\.\d*)?)F\b/g;
  return s.replace(test, convert);
}

Specifications

Specification
ECMAScript (ECMA-262)
The definition of 'String.prototype.replace' in that specification.

Browser compatibility

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DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
replaceChrome Full support 1Edge Full support 12Firefox Full support 1IE Full support 4Opera Full support 4Safari Full support 1WebView Android Full support 1Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 10.1Safari iOS Full support 1Samsung Internet Android Full support 1.0nodejs Full support 0.1.100

Legend

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Full support

See also