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.

Syntax

str.replace(regexp|substr, newSubStr|function[, flags])

Parameters

regexp (pattern)

A RegExp object or literal. The match is replaced by the return value of parameter #2.

substr (pattern)

A String that is to be replaced by newSubStr. It is treated as a verbatim string and is not interpreted as a regular expression.

newSubStr (replacement)

The String that replaces the substring received from parameter #1. 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 put in place of the substring received from parameter #1). The arguments supplied to this function are described in the "Specifying a function as a parameter" section below.

flags

Note: The flags argument does not work in v8 Core (Chrome and Node.js) and will be removed from Firefox. A string specifying a combination of regular expression flags. The use of the flags parameter in the String.prototype.replace() method is non-standard and deprecated. Instead of using this parameter, use a RegExp object or literal with the corresponding flags. The value of this parameter if it is used should be a string consisting of one or more of the following characters to affect the operation as described:

g

global match

i

ignore case

m

match over multiple lines

y

sticky

Return value

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

Description

This method does not change the String object it is called on. 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:

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:

(The exact number of arguments will depend on whether the first argument was 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(' - ');
}
var newString = 'abc12345#$*%'.replace(/([^\d]*)(\d*)([^\w]*)/, replacer);

Examples

Defining the regular expression in replace()

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

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

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

Using global and ignore with replace()

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'.

var re = /apples/gi;
var str = 'Apples are round, and apples are juicy.';
var 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 the $1 and $2 replacement patterns.

var re = /(\w+)\s(\w+)/;
var str = 'John Smith';
var 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) {
    return '-' + 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.

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

This is because '$&'.toLowerCase() would be evaluated first as a string literal (resulting in the same '$&') 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';
  }
  var s = String(x);
  var test = /(-?\d+(?:\.\d*)?)F\b/g;
  return s.replace(test, convert);
}

Use an inline function with a regular expression to avoid for loops

The following example takes a string pattern and converts it into an array of objects.

Input:

A string made out of the characters x, - and _

x-x_
x---x---x---x---
x-xxx-xx-x-
x_x_x___x___x___

Output:

An array of objects. An 'x' denotes an 'on' state, a '-' (hyphen) denotes an 'off' state and an '_' (underscore) denotes the length of an 'on' state.

[
  { on: true, length: 1 },
  { on: false, length: 1 },
  { on: true, length: 2 }
  ...
]

Snippet:

var str = 'x-x_';
var retArr = [];
str.replace(/(x_*)|(-)/g, function(match, p1, p2) {
  if (p1) { retArr.push({ on: true, length: p1.length }); }
  if (p2) { retArr.push({ on: false, length: 1 }); }
});

console.log(retArr);

This snippet generates an array of 3 objects in the desired format without using a for loop.

Specifications

Browser compatibility

Firefox-specific notes

• Starting with Gecko 27 (Firefox 27 / Thunderbird 27 / SeaMonkey 2.24), this method has been adjusted to conform with the ECMAScript specification. When replace() is called with a global regular expression, the RegExp.lastIndex property (if specified) will be reset to 0 (bug 501739).
• Starting with Gecko 39 (Firefox 39 / Thunderbird 39 / SeaMonkey 2.36), the non-standard flags argument is deprecated and throws a console warning (bug 1142351).
• Starting with Gecko 47 (Firefox 47 / Thunderbird 47 / SeaMonkey 2.44), the non-standard flags argument is no longer supported in non-release builds and will soon be removed entirely (bug 1245801).

See also

• String.prototype.match()
• RegExp.prototype.exec()
• RegExp.prototype.test()