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; find all matches rather than stopping after the first 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
lastIndexproperty 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 {{jsxref("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 Sets
- Boundaries
- Alternation
- Grouping and back references
- Quantifiers
- Assertions
| Character Classes | |
|---|---|
| Character | Meaning |
. |
(The dot, the decimal point) matches any single character except line terminators: Inside a character set, the dot loses its special meaning and matches a literal dot. Note that the For example, |
\d |
Matches a digit character in the basic Latin alphabet. Equivalent to For example, |
\D |
Matches any character that is not a digit in the basic Latin alphabet. Equivalent to For example, |
\w |
Matches any alphanumeric character from the basic Latin alphabet, including the underscore. Equivalent to For example, |
\W |
Matches any character that is not a word character from the basic Latin alphabet. Equivalent to For example, |
\s |
Matches a single white space character, including space, tab, form feed, line feed and other Unicode spaces. Equivalent to For example, |
\S |
Matches a single character other than white space. Equivalent to For example, |
\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 For example, |
\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, 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, |
| Character Sets | |
| Character | Meaning |
[xyz] |
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, |
|
|
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, |
| Alternation | |
| Character | Meaning |
x|y |
Matches either For example, |
| 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, |
$ |
Matches end of input. If the multiline flag is set to true, also matches immediately before a line break character. For example, |
\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: |
\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, |
| Grouping and back references | |
| Character | Meaning |
(x) |
Matches For example, 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 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 For example, |
(?: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, |
x+ |
Matches the preceding item x 1 or more times. Equivalent to For example, |
x? |
Matches the preceding item x 0 or 1 time. For example, If used immediately after any of the quantifiers |
x{n} |
Where For example, |
x{n,} |
Where For example, |
x{n,m} |
Where For example, |
|
|
Matches the preceding item x like For example, Quantifiers without |
| Assertions | |
| Character | Meaning |
x(?=y) |
Matches For example, / |
x(?!y) |
Matches For example, |
Properties
- {{jsxref("RegExp.prototype")}}
- Allows the addition of properties to all objects.
RegExp.length- The value of
RegExp.lengthis 2. - {{jsxref("RegExp.@@species", "get RegExp[@@species]")}}
- The constructor function that is used to create derived objects.
- {{jsxref("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
Methods
Examples
Using a regular expression to change data format
The following script uses the {{jsxref("String.prototype.replace()", "replace()")}} method of the {{jsxref("Global_Objects/String", "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 {{jsxref("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 = 'https://xxx.domain.com'; console.log(/[^.]+/.exec(url)[0].substr(7)); // logs 'xxx'
Specifications
| Specification | Status | Comment |
|---|---|---|
| {{SpecName('ES3')}} | {{Spec2('ES3')}} | Initial definition. Implemented in JavaScript 1.1. |
| {{SpecName('ES5.1', '#sec-15.10', 'RegExp')}} | {{Spec2('ES5.1')}} | |
| {{SpecName('ES6', '#sec-regexp-regular-expression-objects', 'RegExp')}} | {{Spec2('ES6')}} | The RegExp constructor no longer throws when the first argument is a RegExp and the second argument is present. Introduces Unicode and sticky flags. |
| {{SpecName('ESDraft', '#sec-regexp-regular-expression-objects', 'RegExp')}} | {{Spec2('ESDraft')}} |
Browser compatibility
| Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
|---|---|---|---|---|---|
| Basic support | {{CompatVersionUnknown}} | {{CompatVersionUnknown}} | {{CompatVersionUnknown}} | {{CompatVersionUnknown}} | {{CompatVersionUnknown}} |
| Sticky flag ("y") | {{CompatChrome("39")}} [1] | {{CompatGeckoDesktop("1.9")}} | {{CompatNo}} | {{CompatNo}} | {{CompatNo}} |
| Unicode flag ("u") | {{CompatChrome("50")}} | {{CompatGeckoDesktop("46")}} | {{CompatUnknown}} | {{CompatUnknown}} | {{CompatUnknown}} |
RegExp(RegExp object, flags) no longer throws |
{{CompatChrome("??")}} | {{CompatGeckoDesktop("39")}} | {{CompatNo}} | {{CompatNo}} | {{CompatNo}} |
| Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
|---|---|---|---|---|---|---|
| Basic support | {{CompatVersionUnknown}} | {{CompatVersionUnknown}} | {{CompatVersionUnknown}} | {{CompatVersionUnknown}} | {{CompatVersionUnknown}} | {{CompatVersionUnknown}} |
| Sticky flag ("y") | {{CompatNo}} | {{CompatNo}} | {{CompatGeckoMobile("1.9")}} | {{CompatNo}} | {{CompatNo}} | {{CompatNo}} |
| Unicode flag ("u") | {{CompatUnknown }} | {{CompatGeckoMobile("46")}} | {{CompatUnknown}} | {{CompatUnknown}} | {{CompatUnknown}} | |
RegExp(RegExp object, flags) no longer throws |
{{CompatNo}} | {{CompatNo}} | {{CompatGeckoMobile("39")}} | {{CompatNo}} | {{CompatNo}} | {{CompatNo}} |
[1] Behind a flag.
Gecko-specific notes
Starting with Gecko 34 {{geckoRelease(34)}}, 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
- Regular Expressions chapter in the JavaScript Guide
- {{jsxref("String.prototype.match()")}}
- {{jsxref("String.prototype.replace()")}}