RegExp

Files
RegExp.js

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

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 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 new lines.)
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 eitherxory`.
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 "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 '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].
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].
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. Equivalent to:
[\t\n\v\f\r \u00a0\u2000\u2001\u2002\u2003\u2004\u2005\u2006\u2007\u2008\u2009\u200a\u200b\u2028\u2029\u3000]
For example, /\s\w*\/ matches ' bar' in "foo bar."
\S Matches a single character other than white space. Equivalent to:
[^\t\n\v\f\r \u00a0\u2000\u2001\u2002\u2003\u2004\u2005\u2006\u2007\u2008\u2009\u200a\u200b\u2028\u2029\u3000]
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 NULL 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.

Documentation for this class comes from MDN and is available under Creative Commons: Attribution-Sharealike license.
Defined By

Properties

RegExp
: Boolean
Whether to test the regular expression against all possible matches in a string, or only against the first. ...

Whether to test the regular expression against all possible matches in a string, or only against the first.

global is a property of an individual regular expression object.

The value of global is true if the "g" flag was used; otherwise, false. The "g" flag indicates that the regular expression should be tested against all possible matches in a string.

You cannot change this property directly.

RegExp
: Boolean

Whether to ignore case while attempting a match in a string.

ignoreCase is a property of an individual regular expression object.

The value of ignoreCase is true if the "i" flag was used; otherwise, false. The "i" flag indicates that case should be ignored while attempting a match in a string.

You cannot change this property directly.

RegExp
: Number

The index at which to start the next match. A read/write integer property that specifies the index at which to start the next match.

lastIndex is a property of an individual regular expression object.

This property is set only if the regular expression used the "g" flag to indicate a global search. The following rules apply:

  • If lastIndex is greater than the length of the string, regexp.test and regexp.exec fail, and lastIndex is set to 0.
  • If lastIndex is equal to the length of the string and if the regular expression matches the empty string, then the regular expression matches input starting at lastIndex.
  • If lastIndex is equal to the length of the string and if the regular expression does not match the empty string, then the regular expression mismatches input, and lastIndex is reset to 0.
  • Otherwise, lastIndex is set to the next position following the most recent match.

For example, consider the following sequence of statements:

  • re = /(hi)?/g Matches the empty string.
  • re("hi") Returns ["hi", "hi"] with lastIndex equal to 2.
  • re("hi") Returns [""], an empty array whose zeroth element is the match string. In this case, the empty string because lastIndex was 2 (and still is 2) and "hi" has length 2.
RegExp
: Boolean

Whether or not to search in strings across multiple lines.

multiline is a property of an individual regular expression object..

The value of multiline is true if the "m" flag was used; otherwise, false. The "m" flag indicates that a multiline input string should be treated as multiple lines. For example, if "m" is used, "^" and "$" change from matching at only the start or end of the entire string to the start or end of any line within the string.

You cannot change this property directly.

RegExp
: String

The text of the pattern.

A read-only property that contains the text of the pattern, excluding the forward slashes.

source is a property of an individual regular expression object.

You cannot change this property directly.

Defined By

Methods

RegExp
new( String pattern, String flags ) : Object

Creates new regular expression object.

Parameters

  • pattern : String

    The text of the regular expression.

  • flags : String

    If specified, flags can have any combination of the following values:

    • "g" - global match
    • "i" - ignore case
    • "m" - 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)

Returns

RegExp
( String str ) : Array

Executes a search for a match in its string parameter.

If the match succeeds, the exec method returns an array and updates properties of the regular expression object. The returned array has the matched text as the first item, and then one item for each capturing parenthesis that matched containing the text that was captured. If the match fails, the exec method returns null.

If you are executing a match simply to find true or false, use the test method or the String search method.

Consider the following example:

// Match one d followed by one or more b's followed by one d
// Remember matched b's and the following d
// Ignore case
var re = /d(b+)(d)/ig;
var result = re.exec("cdbBdbsbz");

The following table shows the results for this script:

Object Property/Index Description Example
result The content of myArray. ["dbBd", "bB", "d"]
index The 0-based index of the match in the string 1
input The original string. cdbDdbsbz
[0] The last matched characters. dbBd
[1], ...[n] The parenthesized substring matches, if any. The number of possible [1] = bB
parenthesized substrings is unlimited. [2] = d
re lastIndex The index at which to start the next match. 5
ignoreCase Indicates the "i" flag was used to ignore case. true
global Indicates the "g" flag was used for a global match. true
multiline Indicates the "m" flag was used to search in strings across false
multiple lines.
source The text of the pattern. d(b+)(d)

If your regular expression uses the "g" flag, you can use the exec method multiple times to find successive matches in the same string. When you do so, the search starts at the substring of str specified by the regular expression's lastIndex property (test will also advance the lastIndex property). For example, assume you have this script:

var myRe = /ab*\/g;
var str = "abbcdefabh";
var myArray;
while ((myArray = myRe.exec(str)) != null)
{
    var msg = "Found " + myArray[0] + ".  ";
    msg += "Next match starts at " + myRe.lastIndex;
print(msg);
}

This script displays the following text:

Found abb. Next match starts at 3
Found ab. Next match starts at 9

You can also use exec() without creating a RegExp object:

var matches = /(hello \S+)/.exec('This is a hello world!');
alert(matches[1]);

This will display an alert containing 'hello world!';

Parameters

  • str : String

    The string against which to match the regular expression.

Returns

  • Array

    Array of results or NULL.

RegExp
( String str ) : Boolean

Tests for a match in its string parameter.

When you want to know whether a pattern is found in a string use the test method (similar to the String.search method); for more information (but slower execution) use the exec method (similar to the String.match method). As with exec (or in combination with it), test called multiple times on the same global regular expression instance will advance past the previous match.

The following example prints a message which depends on the success of the test:

function testinput(re, str){
    if (re.test(str))
        midstring = " contains ";
    else
        midstring = " does not contain ";
    document.write (str + midstring + re.source);
}

Parameters

  • str : String

    The string against which to match the regular expression.

Returns

  • Boolean

    true if string contains any matches, otherwise returns false.

RegExp
( ) : String

Returns a string representing the specified object. Overrides the Object.prototype.toString method.

The RegExp object overrides the toString method of the Object object; it does not inherit Object.toString. For RegExp objects, the toString method returns a string representation of the regular expression.

The following example displays the string value of a RegExp object:

myExp = new RegExp("a+b+c");
alert(myExp.toString());       // displays "/a+b+c/"

Returns

  • String

    Regular expression as a string.