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.

regexp (pattern)

A RegExp object or literal. The match or matches are replaced with newSubstr or the value returned by the specified replacerFunction.

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.

If newSubstr is an empty string, then the substring given by substr, or the matches for regexp, are removed (rather then being replaced).

replacerFunction (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.

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

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

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

The replacement string can include the following special replacement patterns:

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 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#$*%'.replace(/([^\d]*)(\d*)([^\w]*)/, replacer);
console.log(newString);  // abc - 12345 - #$*%

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...'.

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'.

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'.

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.

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);
}

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