RegExp

Summary

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

For an introduction on what  regular expressions are, read the Regular Expressions chapter in the JavaScript Guide.

Constructor

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)
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). This allows the match-only-at-start capabilities of the character "^" to effectively be used at any location in a string by changing the value of the lastIndex property.

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");

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.

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 the newline characters: \n \r \u2028 or \u2029.

Note that the m multiline flag doesn't change the dot behavior. So to match a pattern accross 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 [ \t\r\n].

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

Matches a single character other than white space (Not whitespace). Equivalent to [^ \t\r\n].

For example, /\S\w*/ matches 'foo' in "foo bar."
\t Matches a 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 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, /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 character set. Matches any one of the enclosed characters. You can specify a range of characters by using a hyphen.

For example, [abcd] is the same as [a-d]. They match the 'b' in "brisket" and the 'c' in "chop".
[^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, [^abc] is the same as [^a-c]. They initially match 'o' in "bacon" and 'h' in "chop."
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 zero-width word boundary, such as between a letter and a space. (Not to be confused with [\b])

For example, /\bno/ matches the 'no' in "at noon"; /ly\b/ matches the 'ly' in "possibly yesterday."
\B

Matches a zero-width non-word boundary, such as between two letters or between two spaces.

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 parentheses.

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

Matches the preceding item 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".
+

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

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

Matches like * and + from above, however the match is the smallest possible match.

For example, /".*?"/ matches '"foo"' in '"foo" "bar"' and does not match '"foo" "bar"' as without the ? behind the *.
?

Matches the preceding item 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).

Also used in lookahead assertions, described under (?=), (?!), and (?:) in this table.
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.
x|y

Matches either x or y.

For example, /green|red/ matches 'green' in "green apple" and 'red' in "red apple."
{n}

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

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."
{n,}

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

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

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

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.

Notes

  1. ^Equivalent to:

    [\t\n\v\f\r \u00a0\u2000\u2001\u2002\u2003\u2004\u2005\u2006\u2007\u2008\u2009\u200a\u200b\u2028\u2029\u3000]

  2. ^Equivalent to:

    [^\t\n\v\f\r \u00a0\u2000\u2001\u2002\u2003\u2004\u2005\u2006\u2007\u2008\u2009\u200a\u200b\u2028\u2029\u3000]

Properties

For properties available on RegExp instances, see Properties of RegExp instances.
RegExp.prototype
Allows the addition of properties to all objects.
RegExp.length
The value of RegExp.length is 2.
Properties inherited from Function:

Methods

For methods available on RegExp instances, see Methods of RegExp instances.

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

Methods inherited from Function:

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.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.lastIndex
The index at which to start the next match.
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.
Properties inherited from Object:

Methods

See also Deprecated RegExp Methods

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.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

Example: 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");
print(newstr);

This displays "Smith, John".

Example: 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'

Example: Using a regular expression with the "sticky" flag

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);
print(match[1]);  // prints "First"
print(regex.lastIndex); // prints 11

var match2 = regex.exec(text);
print(match2[1]); // prints "Second"
print(regex.lastIndex); // prints "22"

var match3 = regex.exec(text);
print(match3 === null); // prints "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; }
alert(supports_sticky); // alerts "true"

Example: 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 languagessuch 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);
print(match[0]);  // prints "Образец"
print(regex.lastIndex);  // prints "7"

var match2 = regex.exec(text);
print(match2[0]);  // prints "на" [did not print "text"]
print(regex.lastIndex);  // prints "15"

// and so on

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

Example: Extracting subdomain name from URL

var url = "http://xxx.domain.com";
print(/[^.]+/.exec(url)[0].substr(7)); // prints "xxx"

Specifications

Specification Status Comment
ECMAScript 1st Edition. Implemented in JavaScript 1.1 Standard Initial definition.
ECMAScript Language Specification 5.1th Edition (ECMA-262) Standard  
ECMAScript Language Specification 6th Edition (ECMA-262) Draft  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
Sticky flag ("y") ? 3.0 (1.9) ? ? ?
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") ? ? 1.0 (1.9) ? ? ?

See also

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