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;
newRegExp(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).
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: 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 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, |
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, 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 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 |
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 Also used in lookahead assertions, described under |
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|y |
Matches either For example, |
x{n} |
Where For example, |
x{n,} |
Where For example, |
x{n,m} |
Where For example, |
Properties
RegExp.prototype
- Allows the addition of properties to all objects.
- RegExp.length
- The value of
RegExp.length
is 2.
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[@@matchAll]()
- Returns all matches of the regular expression against a string.
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
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");
console.log(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);
console.log(match[1]); // prints "First"
console.log(regex.lastIndex); // prints 11
var match2 = regex.exec(text);
console.log(match2[1]); // prints "Second"
console.log(regex.lastIndex); // prints "22"
var match3 = regex.exec(text);
console.log(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 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]); // prints "Образец"
console.log(regex.lastIndex); // prints "7"
var match2 = regex.exec(text);
console.log(match2[0]); // prints "на" [did not print "text"]
console.log(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";
console.log(/[^.]+/.exec(url)[0].substr(7)); // prints "xxx"
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 1st Edition. Implemented in JavaScript 1.1 | Standard | Initial definition. |
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 |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Sticky flag ("y") | 39 (behind flag) | 3.0 (1.9) ES4-Style bug 773687 | 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 | 3.0 (1.9) ES4-Style bug 773687 | No support | No support | No support |