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Core JavaScript 1.5 Reference:Global Objects:RegExp

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Contents

Summary

Creates a regular expression object for matching text according to a pattern.

Syntax 

var regex = new RegExp("pattern" [, "flags"]);
var literal = /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
match over multiple lines
y
New in Firefox 3
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

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

Notice that the parameters to the literal format do not use quotation marks to indicate strings, 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");

Special characters in regular expressions

Character Meaning
\

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 the it with a backslash; for example, /a\*/ matches 'a*'.

^

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".

*

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 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.

.

(The decimal point) matches any single character except the newline characters: \n \r \u2028 or \u2029. ([\s\S] can be used to match any character including newlines.)

For example, /.n/ matches 'an' and 'on' in "nay, an apple is on the tree", but not 'nay'.

(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.

(?: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.

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 first 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.

[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 "ache".

[^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 'r' in "brisket" and 'h' in "chop."

[\b]

Matches a backspace. (Not to be confused with \b.)

\b

Matches a word boundary, such as a space. (Not to be confused with [\b].)

For example, /\bn\w/ matches the 'no' in "noonday"; /\wy\b/ matches the 'ly' in "possibly yesterday."

\B

Matches a non-word boundary.

For example, /\w\Bn/ matches 'on' in "noonday", and /y\B\w/ matches 'ye' in "possibly yesterday."

\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.

\d

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

Note: In Firefox 2 and earlier, matches a digit character from any alphabet. (bug 378738)

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

\D

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

Note: In Firefox 2 and earlier, all alphabet. (bug 378738)

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

\f

Matches a form-feed.

\n

Matches a linefeed.

\r

Matches a carriage return.

\s

Matches a single white space character, including space, tab, form feed, line feed and other unicode spaces.[1]

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

\S

Matches a single character other than white space.[2]

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

\t

Matches a tab.

\v

Matches a vertical tab.

\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%."

\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.

\0

Matches a NUL character. Do not follow this with another digit.

\xhh

Matches the character with the code hh (two hexadecimal digits)

\uhhhh

Matches the character with the Unicode value hhhh (four hexadecimal digits).

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.

  1. ^ Equivalent to:

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

  1. ^ 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 inherited by RegExp instances, see Properties of RegExp instances.

prototype
Allows the addition of properties to all objects.

Properties inherited from Function.prototype
caller, constructor, length, name

Methods

For methods inherited by 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.prototype
apply, call, toSource, toString, valueOf

Methods inherited from Object.prototype
__defineGetter__, __defineSetter__, hasOwnProperty, isPrototypeOf, __lookupGetter__, __lookupSetter__, __noSuchMethod__, propertyIsEnumerable, unwatch, watch

RegExp instances

RegExp instances inherit from RegExp.prototype. Modifications to the prototype object are propagated to all RegExp instances.

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.

constructor
Specifies the function that creates an object's prototype.
global
Whether to test the regular expression against all possible matches in a string, or only against the first.
ignoreCase
Whether to ignore case while attempting a match in a string.
lastIndex
The index at which to start the next match.
multiline
Whether or not to search in strings across multiple lines.
source
The text of the pattern.

Methods

exec
Executes a search for a match in its string parameter.
test
Tests for a match in its string parameter.
toSource
Non-standard
Returns an object literal representing the specified object; you can use this value to create a new object. Overrides the Object.prototype.toSource method.
toString
Returns a string representing the specified object. Overrides the Object.prototype.toString method.

Methods inherited from Object.prototype
__defineGetter__, __defineSetter__, hasOwnProperty, isPrototypeOf, __lookupGetter__, __lookupSetter__, __noSuchMethod__, propertyIsEnumerable, unwatch, valueOf, watch

Examples

Example: Using a regular expression to change data format

The following script uses the replace method inherited by 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 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 "false" in Firefox 2, "true" in Firefox 3

See also

Retrieved from "http://developer.mozilla.org/en/docs/Core_JavaScript_1.5_Reference:Global_Objects:RegExp"