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
replace(regexp, newSubstr)
replace(regexp, replacerFunction)
replace(substr, newSubstr)
replace(substr, replacerFunction)
Parameters
regexp
(pattern)-
A
RegExp
object or literal. The match or matches are replaced withnewSubstr
or the value returned by the specifiedreplacerFunction
. 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
orsubstr
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 bysubstr
, or the matches forregexp
, 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
orsubstr
. 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 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 n th parenthesized submatch string, provided the first argument was a RegExp object. Note that this is 1 -indexed. If a group n is not present (e.g., if group is 3), it will be replaced as a literal (e.g., $3 ). |
$<Name> |
Where Name is a capturing group name. If the group is not in the match, or not in the regular expression, or if a string was passed as the first argument to replace instead of a regular expression, this resolves to a literal (e.g., $<Name> ). Only available in browser versions supporting named capturing groups. |
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 (including named capturing groups), provided the first argument to replace() was a RegExp object. (Corresponds to $1 , $2 , etc. above.) For example, if /(\a+)(\b+)/ , was given, p1 is the match for \a+ , and p2 for \b+ . |
offset |
The offset of the matched substring within the whole string being examined. (For example, if the whole string was 'abcd' , and the matched substring was 'bc' , then this argument will be 1 .) |
string |
The whole string being examined. |
groups |
In browser versions supporting named capturing groups, will be an object whose keys are the used group names, and whose values are the matched portions (undefined if not matched). |
(The exact number of arguments depends on whether the first argument is a
RegExp
object—and, if so, how many parenthesized submatches it specifies.)
The following example will set newString
to 'abc - 12345 - #$*%'
:
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 Language Specification # sec-string.prototype.replace |
Browser compatibility
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