pattern

Can be a string or an object with a Symbol.replace method — the typical example being a regular expression. Any value that doesn't have the Symbol.replace method will be coerced to a string.

replacement

Can be a string or a function.

• If it's a string, it will replace the substring matched by pattern. A number of special replacement patterns are supported; see the Specifying a string as the replacement section below.
• If it's a function, it will be invoked for every match and its return value is used as the replacement text. The arguments supplied to this function are described in the Specifying a function as the replacement section below.

A new string, with one, some, or all matches of the pattern replaced by the specified replacement.

This method does not mutate the string value it's called on. It returns a new string.

A string pattern will only be replaced once. To perform a global search and replace, use a regular expression with the g flag, or use replaceAll() instead.

If pattern is an object with a Symbol.replace method (including RegExp objects), that method is called with the target string and replacement as arguments. Its return value becomes the return value of replace(). In this case the behavior of replace() is entirely encoded by the @@replace method — for example, any mention of "capturing groups" in the description below is actually functionality provided by RegExp.prototype[@@replace].

If the pattern is an empty string, the replacement is prepended to the start of the string.

"xxx".replace("", "_"); // "_xxx"

A regexp with the g flag is the only case where replace() replaces more than once. For more information about how regex properties (especially the sticky flag) interact with replace(), see RegExp.prototype[@@replace]().

The replacement string can include the following special replacement patterns:

$n and $<Name> are only available if the pattern argument is a RegExp object. If the pattern is a string, or if the corresponding capturing group isn't present in the regex, then the pattern will be replaced as a literal. If the group is present but isn't matched (because it's part of a disjunction), it will be replaced with an empty string.

"foo".replace(/(f)/, "$2"); // "$2oo"; the regex doesn't have the second group
"foo".replace("f", "$1"); // "$1oo"; the pattern is a string, so it doesn't have any groups
"foo".replace(/(f)|(g)/, "$2"); // "oo"; the second group exists but isn't matched

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.

The function has the following signature:

function replacer(match, p1, p2, /* …, */ pN, offset, string, groups) {
  return replacement;
}

The arguments to the function are as follows:

match

The matched substring. (Corresponds to $& above.)

p1, p2, …, pN

The nth string found by a capture group (including named capturing groups), provided the first argument to replace() is a RegExp object. (Corresponds to $1, $2, etc. above.) For example, if the pattern is /(\a+)(\b+)/, then p1 is the match for \a+, and p2 is the match for \b+. If the group is part of a disjunction (e.g. "abc".replace(/(a)|(b)/, replacer)), the unmatched alternative will be undefined.

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.

groups

An object whose keys are the used group names, and whose values are the matched portions (undefined if not matched). Only present if the pattern contains at least one named capturing group.

The exact number of arguments depends on whether the first argument is a RegExp object — and, if so, how many capture groups it has.

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

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

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

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

const str = 'Twas the night before Xmas...';
const 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'.

const re = /apples/gi;
const str = 'Apples are round, and apples are juicy.';
const 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.

const re = /(\w+)\s(\w+)/;
const str = 'John Smith';
const 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.

const 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 degrees is accessible to the function through its second parameter, p1. The function sets the Celsius number based on the number of Fahrenheit degrees 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`;
  }
  const s = String(x);
  const test = /(-?\d+(?:\.\d*)?)F\b/g;
  return s.replace(test, convert);
}

Suppose we want to create a replacer that appends the offset data to every matched string. Because the replacer function already receives the offset parameter, it will be trivial if the regex is statically known.

console.log("abcd".replace(/(bc)/, (match, p1, offset) => `${match} (${offset}) `)); // "abc (1) d"

However, this replacer would be hard to generalize if we want it to work with any regex pattern. The replacer is variadic — the number of arguments it receives depends on the number of capturing groups present. We can use rest parameters, but it would also collect offset, string, etc. into the array. The fact that groups may or may not be passed depending on the identity of the regex would also make it hard to generically know which argument corresponds to the offset.

function addOffset(match, ...args) {
  const offset = args.at(-2);
  return `${match} (${offset}) `;
}

console.log("abcd".replace(/(bc)/, addOffset)); // "abc (1) d"
console.log("abcd".replace(/(?<group>bc)/, addOffset)); // "abc (abcd) d"

The addOffset example above doesn't work when the regex contains a named group, because in this case args.at(-2) would be the string instead of the offset.

Instead, you need to extract the last few arguments based on type, because groups is an object while string is a string.

function addOffset(match, ...args) {
  const hasNamedGroups = typeof args.at(-1) === "object";
  const offset = hasNamedGroups ? args.at(-3) : args.at(-2);
  return `${match} (${offset}) `;
}

console.log("abcd".replace(/(bc)/, addOffset)); // "abc (1) d"
console.log("abcd".replace(/(?<group>bc)/, addOffset)); // "abc (1) d"

• Polyfill of String.prototype.replace in core-js with fixes and implementation of modern behavior like Symbol.replace support
• String.prototype.replaceAll()
• String.prototype.match()
• RegExp.prototype.exec()
• RegExp.prototype.test()
• Symbol.replace
• RegExp.prototype[@@replace]()