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.
Literal and constructor notations are possible:
/pattern/flags
new RegExp(pattern[, flags])
pattern
flags
If specified, flags can have any combination of the following values:
g
i
m
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+');
Character Classes | |
---|---|
Character | Meaning |
. |
(The dot, the decimal point) matches any single character except line terminators: Inside character class, 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 the character with the Unicode value hhhh (four 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. For example, |
[^xyz] |
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. 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 zero-width word boundary, such as between a letter and a space. (Not to be confused with For example, |
\B |
Matches a zero-width non-word boundary, such as between two letters or between two spaces. 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*? x+? |
Matches the preceding item x like 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, |
Assertions | |
Character | Meaning |
x(?=y) |
Matches For example, / |
x(?!y) |
Matches For example, |
RegExp.length
RegExp.length
is 2.The global RegExp
object has no methods of its own, however, it does inherit some methods through the prototype chain.
RegExp
prototype objects and instancesThe 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".
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.
var s = 'Please yes\nmake my day!'; s.match(/yes.*day/); // Returns null s.match(/yes[^]*day/); // Returns 'yes\nmake my day'
This example demonstrates how one could use the sticky flag on regular expressions to match individual lines of multiline input.
var text = 'First line\nSecond line'; var regex = /(\S+) line\n?/y; var match = regex.exec(text); console.log(match[1]); // logs 'First' console.log(regex.lastIndex); // logs '11' var match2 = regex.exec(text); console.log(match2[1]); // logs 'Second' console.log(regex.lastIndex); // logs '22' var match3 = regex.exec(text); console.log(match3 === null); // logs 'true'
One can test at run-time whether the sticky flag is supported, using try { … } catch { … }
. For this, either an eval(…)
expression or the RegExp(regex-string, flags-string)
syntax must be used (since the /regex/flags
notation is processed at compile-time, so throws an exception before the catch
block is encountered). For example:
var supports_sticky; try { RegExp('', 'y'); supports_sticky = true; } catch(e) { supports_sticky = false; } console.log(supports_sticky); // logs 'true'
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.
var url = 'http://xxx.domain.com'; console.log(/[^.]+/.exec(url)[0].substr(7)); // logs 'xxx'
[1] Behind a flag.
Starting with Gecko 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
().
Created by Mozilla Contributors and licensed under CC-BY-SA 2.5