The RegExp constructor creates a regular expression object for matching text with a pattern.

For an introduction to regular expressions, read the Regular Expressions chapter in the JavaScript Guide.

Syntax

Literal and constructor notations are possible:

/pattern/flags
new RegExp(pattern[, flags])

Parameters

pattern
The text of the regular expression.
flags

If specified, flags can have any combination of the following values:

g
global match
i
ignore case
m
multiline; treat beginning and end characters (^ and $) as working over multiple lines (i.e., match the beginning or end of each line (delimited by \n or \r), not only the very beginning or end of the whole input string)
u
unicode; treat pattern as a sequence of unicode code points
y
sticky; matches only from the index indicated by the lastIndex property of this regular expression in the target string (and does not attempt to match from any later indexes).

Description

There are 2 ways to create a RegExp object: a literal notation and a constructor. To indicate strings, the parameters to the literal notation do not use quotation marks while the parameters to the constructor function do use quotation marks. So the following expressions create the same regular expression:

/ab+c/i;
new RegExp('ab+c', 'i');
new RegExp(/ab+c/, 'i');

The literal notation provides compilation of the regular expression when the expression is evaluated. Use literal notation when the regular expression will remain constant. For example, if you use literal notation to construct a regular expression used in a loop, the regular expression won't be recompiled on each iteration.

The constructor of the regular expression object, for example, new RegExp('ab+c'), provides runtime compilation of the regular expression. Use the constructor function when you know the regular expression pattern will be changing, or you don't know the pattern and are getting it from another source, such as user input.

Starting with ECMAScript 6, new RegExp(/ab+c/, 'i') no longer throws a TypeError ("can't supply flags when constructing one RegExp from another") when the first argument is a RegExp and the second flags argument is present. A new RegExp from the arguments is created instead.

When using the constructor function, the normal string escape rules (preceding special characters with \ when included in a string) are necessary. For example, the following are equivalent:

var re = /\w+/;
var re = new RegExp('\\w+');

Special characters meaning in regular expressions

Character Classes
Character Meaning
.

(The dot, the decimal point) matches any single character except line terminators: \n, \r, \u2028 or \u2029.

Inside a character class, the dot loses its special meaning and matches a literal dot.

Note that the m multiline flag doesn't change the dot behavior. So to match a pattern across multiple lines, the character set [^] can be used (if you don't mean an old version of IE, of course), it will match any character including newlines.

For example, /.y/ matches "my" and "ay", but not "yes", in "yes make my day".
\d

Matches a digit character in the basic Latin alphabet. Equivalent to [0-9].

For example, /\d/ or /[0-9]/ matches "2" in "B2 is the suite number".
\D

Matches any character that is not a digit in the basic Latin alphabet. Equivalent to [^0-9].

For example, /\D/ or /[^0-9]/ matches "B" in "B2 is the suite number".
\w

Matches any alphanumeric character from the basic Latin alphabet, including the underscore. Equivalent to [A-Za-z0-9_].

For example, /\w/ matches "a" in "apple", "5" in "$5.28", and "3" in "3D".
\W

Matches any character that is not a word character from the basic Latin alphabet. Equivalent to [^A-Za-z0-9_].

For example, /\W/ or /[^A-Za-z0-9_]/ matches "%" in "50%".
\s

Matches a single white space character, including space, tab, form feed, line feed and other Unicode spaces. Equivalent to [ \f\n\r\t\v\u00a0\u1680\u180e\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff].

For example, /\s\w*/ matches " bar" in "foo bar".
\S

Matches a single character other than white space. Equivalent to [^ \f\n\r\t\v\u00a0\u1680\u180e\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff].

For example, /\S\w*/ matches "foo" in "foo bar".
\t Matches a horizontal tab.
\r Matches a carriage return.
\n Matches a linefeed.
\v Matches a vertical tab.
\f Matches a form-feed.
[\b] Matches a backspace. (Not to be confused with \b)
\0 Matches a NUL character. Do not follow this with another digit.
\cX

Where X is a letter from A - Z. Matches a control character in a string.

For example, /\cM/ matches control-M in a string.
\xhh Matches the character with the code hh (two hexadecimal digits).
\uhhhh Matches a UTF-16 code-unit with the value hhhh (four hexadecimal digits).
\u{hhhh} or \u{hhhhh} (only when u flag is set) Matches the character with the Unicode value U+hhhh or U+hhhhh (hexadecimal digits).
\

For characters that are usually treated literally, indicates that the next character is special and not to be interpreted literally.

For example, /b/ matches the character "b". By placing a backslash in front of "b", that is by using /\b/, the character becomes special to mean match a word boundary.

or

For characters that are usually treated specially, indicates that the next character is not special and should be interpreted literally.

For example, "*" is a special character that means 0 or more occurrences of the preceding character should be matched; for example, /a*/ means match 0 or more "a"s. To match * literally, precede it with a backslash; for example, /a\*/ matches "a*".
Character Sets
Character Meaning
[xyz]
[a-c]

A character set. Matches any one of the enclosed characters. You can specify a range of characters by using a hyphen, but if the hyphen appears as the first or last character enclosed in the square brackets it is taken as a literal hyphen to be included in the character set as a normal character.

For example, [abcd] is the same as [a-d]. They match the "b" in "brisket" and the "c" in "chop".

[^xyz]
[^a-c]

A negated or complemented character set. That is, it matches anything that is not enclosed in the brackets. You can specify a range of characters by using a hyphen, but if the hyphen appears as the first or last character enclosed in the square brackets it is taken as a literal hyphen to be included in the character set as a normal character.

For example, [^abc] is the same as [^a-c]. They initially match "o" in "bacon" and "h" in "chop".
Alternation
Character Meaning
x|y

Matches either x or y.

For example, /green|red/ matches "green" in "green apple" and "red" in "red apple".
Boundaries
Character Meaning
^

Matches beginning of input. If the multiline flag is set to true, also matches immediately after a line break character.

For example, /^A/ does not match the "A" in "an A", but does match the first "A" in "An A".
$

Matches end of input. If the multiline flag is set to true, also matches immediately before a line break character.

For example, /t$/ does not match the "t" in "eater", but does match it in "eat".
\b

Matches a word boundary. This is the position where a word character is not followed or preceeded by another word-character, such as between a letter and a space. Note that a matched word boundary is not included in the match. In other words, the length of a matched word boundary is zero.

Examples:
/\bm/ matches the 'm' in "moon" ;
/oo\b/ does not match the 'oo' in "moon", because 'oo' is followed by 'n' which is a word character;
/oon\b/ matches the 'oon' in "moon", because 'oon' is the end of the string, thus not followed by a word character;
/\w\b\w/ will never match anything, because a word character can never be followed by both a non-word and a word character.
\B

Matches a non-word boundary. This is a position where the previous and next character are of the same type: Either both must be words, or both must be non-words. Such as between two letters or between two spaces. The beginning and end of a string are considered non-words. Same as the matched word boundary, the matched non-word bondary is also not included in the match.

For example, /\Bon/ matches "on" in "at noon", and /ye\B/ matches "ye" in "possibly yesterday".
Grouping and back references
Character Meaning
(x)

Matches x and remembers the match. These are called capturing groups.

For example, /(foo)/ matches and remembers "foo" in "foo bar". 

The capturing groups are numbered according to the order of left parentheses of capturing groups, starting from 1. The matched substring can be recalled from the resulting array's elements [1], ..., [n] or from the predefined RegExp object's properties $1, ..., $9.

Capturing groups have a performance penalty. If you don't need the matched substring to be recalled, prefer non-capturing parentheses (see below).
\n

Where n is a positive integer. A back reference to the last substring matching the n parenthetical in the regular expression (counting left parentheses).

For example, /apple(,)\sorange\1/ matches "apple, orange," in "apple, orange, cherry, peach". A more complete example follows this table.
(?:x) Matches x but does not remember the match. These are called non-capturing groups. The matched substring can not be recalled from the resulting array's elements [1], ..., [n] or from the predefined RegExp object's properties $1, ..., $9.
Quantifiers
Character Meaning
x*

Matches the preceding item x 0 or more times.

For example, /bo*/ matches "boooo" in "A ghost booooed" and "b" in "A bird warbled", but nothing in "A goat grunted".
x+

Matches the preceding item x 1 or more times. Equivalent to {1,}.

For example, /a+/ matches the "a" in "candy" and all the "a"'s in "caaaaaaandy".
x?

Matches the preceding item x 0 or 1 time.

For example, /e?le?/ matches the "el" in "angel" and the "le" in "angle."

If used immediately after any of the quantifiers *, +, ?, or {}, makes the quantifier non-greedy (matching the minimum number of times), as opposed to the default, which is greedy (matching the maximum number of times).
x{n}

Where n is a positive integer. Matches exactly n occurrences of the preceding item x.

For example, /a{2}/ doesn't match the "a" in "candy", but it matches all of the "a"'s in "caandy", and the first two "a"'s in "caaandy".
x{n,}

Where n is a positive integer. Matches at least n occurrences of the preceding item x.

For example, /a{2,}/ doesn't match the "a" in "candy", but matches all of the a's in "caandy" and in "caaaaaaandy".
x{n,m}

Where n and m are positive integers. Matches at least n and at most m occurrences of the preceding item x.

For example, /a{1,3}/ matches nothing in "cndy", the "a" in "candy", the two "a"'s in "caandy", and the first three "a"'s in "caaaaaaandy". Notice that when matching "caaaaaaandy", the match is "aaa", even though the original string had more "a"'s in it.

x*?
x+?
x??
x{n}?
x{n,}?
x{n,m}?

Matches the preceding item x like *, +, ?, and {...} from above, however the match is the smallest possible match.

For example, /<.*?>/ matches "<foo>" in "<foo> <bar>", whereas /<.*>/ matches "<foo> <bar>".

Quantifiers without ? are said to be greedy. Those with ? are called "non-greedy".
Assertions
Character Meaning
x(?=y)

Matches x only if x is followed by y.

For example, /Jack(?=Sprat)/ matches "Jack" only if it is followed by "Sprat".
/Jack(?=Sprat|Frost)/ matches "Jack" only if it is followed by "Sprat" or "Frost". However, neither "Sprat" nor "Frost" is part of the match results.
x(?!y)

Matches x only if x is not followed by y.

For example, /\d+(?!\.)/ matches a number only if it is not followed by a decimal point.
/\d+(?!\.)/.exec('3.141') matches "141" but not "3.141".

Properties

RegExp.prototype
Allows the addition of properties to all objects.
RegExp.length
The value of RegExp.length is 2.
get RegExp[@@species]
The constructor function that is used to create derived objects.
RegExp.lastIndex
The index at which to start the next match.

Methods

The global RegExp object has no methods of its own, however, it does inherit some methods through the prototype chain.

RegExp prototype objects and instances

Properties

See also deprecated RegExp properties.

Note that several of the RegExp properties have both long and short (Perl-like) names. Both names always refer to the same value. Perl is the programming language from which JavaScript modeled its regular expressions.

RegExp.prototype.constructor
Specifies the function that creates an object's prototype.
RegExp.prototype.flags
A string that contains the flags of the RegExp object.
RegExp.prototype.global
Whether to test the regular expression against all possible matches in a string, or only against the first.
RegExp.prototype.ignoreCase
Whether to ignore case while attempting a match in a string.
RegExp.prototype.multiline
Whether or not to search in strings across multiple lines.
RegExp.prototype.source
The text of the pattern.
RegExp.prototype.sticky
Whether or not the search is sticky.
RegExp.prototype.unicode
Whether or not Unicode features are enabled.

Methods

RegExp.prototype.compile()
(Re-)compiles a regular expression during execution of a script.
RegExp.prototype.exec()
Executes a search for a match in its string parameter.
RegExp.prototype.test()
Tests for a match in its string parameter.
RegExp.prototype[@@match]()
Performs match to given string and returns match result.
RegExp.prototype[@@replace]()
Replaces matches in given string with new substring.
RegExp.prototype[@@search]()
Searches the match in given string and returns the index the pattern found in the string.
RegExp.prototype[@@split]()
Splits given string into an array by separating the string into substring.
RegExp.prototype.toSource()
Returns an object literal representing the specified object; you can use this value to create a new object. Overrides the Object.prototype.toSource() method.
RegExp.prototype.toString()
Returns a string representing the specified object. Overrides the Object.prototype.toString() method.

Examples

Using a regular expression to change data format

The following script uses the replace() method of the String instance to match a name in the format first last and output it in the format last, first. In the replacement text, the script uses $1 and $2 to indicate the results of the corresponding matching parentheses in the regular expression pattern.

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

This displays "Smith, John".

Using regular expression to split lines with different line endings/ends of line/line breaks

The default line ending varies depending on the platform (Unix, Windows, etc.). The line splitting provided in this example works on all platforms.

var text = 'Some text\nAnd some more\r\nAnd yet\rThis is the end';
var lines = text.split(/\r\n|\r|\n/);
console.log(lines); // logs [ 'Some text', 'And some more', 'And yet', 'This is the end' ]

Note that the order of the patterns in the regular expression matters.

Using regular expression on multiple lines

var s = 'Please yes\nmake my day!';
s.match(/yes.*day/);
// Returns null
s.match(/yes[^]*day/);
// Returns ["yes\nmake my day"]

Using a regular expression with the sticky flag

The sticky flag indicates that the regular expression performs sticky matching in the target string by attempting to match starting at RegExp.prototype.lastIndex.

var str = '#foo#';
var regex = /foo/y;

regex.lastIndex = 1;
regex.test(str); // true
regex.lastIndex = 5;
regex.test(str); // false (lastIndex is taken into account with sticky flag)
regex.lastIndex; // 0 (reset after match failure)

Regular expression and Unicode characters

As mentioned above, \w or \W only matches ASCII based characters; for example, "a" to "z", "A" to "Z", "0" to "9" and "_". To match characters from other languages such as Cyrillic or Hebrew, use \uhhhh, where "hhhh" is the character's Unicode value in hexadecimal. This example demonstrates how one can separate out Unicode characters from a word.

var text = 'Образец text на русском языке';
var regex = /[\u0400-\u04FF]+/g;

var match = regex.exec(text);
console.log(match[0]);        // logs 'Образец'
console.log(regex.lastIndex); // logs '7'

var match2 = regex.exec(text);
console.log(match2[0]);       // logs 'на' [did not log 'text']
console.log(regex.lastIndex); // logs '15'

// and so on

Here's an external resource for getting the complete Unicode block range for different scripts: Regexp-unicode-block.

Extracting sub-domain name from URL

var url = 'http://xxx.domain.com';
console.log(/[^.]+/.exec(url)[0].substr(7)); // logs 'xxx'

Specifications

Specification Status Comment
ECMAScript 3rd Edition (ECMA-262) Standard Initial definition. Implemented in JavaScript 1.1.
ECMAScript 5.1 (ECMA-262)
The definition of 'RegExp' in that specification.		 Standard  
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'RegExp' in that specification.		 Standard The RegExp constructor no longer throws when the first argument is a RegExp and the second argument is present. Introduces Unicode and sticky flags.
ECMAScript 2017 Draft (ECMA-262)
The definition of 'RegExp' in that specification.		 Draft  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
Sticky flag ("y") 39 [1] 3.0 (1.9) No support No support No support
Unicode flag ("u") 50 46 (46) ? ? ?
RegExp(RegExp object, flags) no longer throws ?? 39 (39) No support No support No support
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)
Sticky flag ("y") No support No support 1.0 (1.9) No support No support No support
Unicode flag ("u") ? 46.0 (46) ? ? ?
RegExp(RegExp object, flags) no longer throws No support No support 39.0 (39) No support No support No support

[1] Behind a flag.

Gecko-specific notes

Starting with Gecko 34 (Firefox 34 / Thunderbird 34 / SeaMonkey 2.31), in the case of a capturing group with quantifiers preventing its exercise, the matched text for a capturing group is now undefined instead of an empty string:

// Firefox 33 or older
'x'.replace(/x(.)?/g, function(m, group) {
  console.log("'group:" + group + "'");
}); // 'group:'

// Firefox 34 or newer
'x'.replace(/x(.)?/g, function(m, group) {
  console.log("'group:" + group + "'");
}); // 'group:undefined'

Note that due to web compatibility, RegExp.$N will still return an empty string instead of undefined (bug 1053944).

See also

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