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RegExp : Object

Pattern for a regular expression.

Platform Support

IE Mozilla Netscape Opera Safari
4.0+ 1.0+ 4.0+ 7.0+ 1.0+

Constructors

Constructor Action IE Mozilla Netscape Opera Safari
RegExp Constructor([String pattern,] [String flags]) : RegExp
Creates a new instance of RegExp.
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RegExp([String pattern,] [String flags]) : RegExp

Creates a new instance of RegExp.

Parameters

String pattern (optional)The text of the regular expression.
String flags (optional)If specified, flags can have any combination of the following values: g - global match, i - ignore case, m - match over multiple lines.

Returns

RegExp
Throws
Throws SyntaxError if pattern is not a legal regular expression or if flags contains characters other than g, i, or m.
Throws TypeError if pattern is a regular expression object and the flags argument is present.

Properties

Property Action IE Mozilla Netscape Opera Safari
$n : String
String corresponding to the (nth) expression of a match.
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Availability

JavaScript 1.2|JScript 3.0

constructor : Object
Specifies the function that creates the RegExp prototype.
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See Also

Object.constructor

Availability

JavaScript 1.2|JScript 3.0|ECMAScript v3

global : Boolean
Whether or not the "g" flag is used with the regular expression.
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Remarks

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.

Availability

JavaScript 1.2|JScript 5.5|ECMAScript v3

ignoreCase : Boolean
Whether or not the "i" flag is used with the regular expression.
Show Details 5.5+ 1.0+ 4.0+ 7.0+ 1.0+
Remarks

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.

Availability

JavaScript 1.2|JScript 5.5|ECMAScript v3

index : Number
Character position of the start of the first match.
Show Details 4.0+ no no no no
Availability

JScript 3.0

input : String
String containing the most recently found regular expression match.
Show Details 4.0+ 1.0+ 4.0+ no no
Availability

JavaScript 1.2|JScript 3.0

lastIndex : Number
A read/write integer property that specifies the index at which to start the next match.
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Remarks

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.
See Also

RegExp.exec|RegExp.test

Availability

JavaScript 1.2|JScript 5.5|ECMAScript v3

lastMatch : String
String containing the last characters of the last regular expression match.
Show Details 5.5+ 1.0+ 4.0+ no no
Availability

JavaScript 1.2|JScript 5.5

lastParen : String
String containing the most recent parenthesized match.
Show Details 5.5+ 1.0+ 4.0+ no no
Availability

JavaScript 1.2|JScript 5.5

leftContext : String
String containing the characters before the most recently found regular expression match.
Show Details 5.5+ 1.0+ 4.0+ no no
Availability

JavaScript 1.2|JScript 5.5

multiline : Boolean
Whether or not the "m" (multiline) flag is used with the regular expression.
Show Details 5.5+ 1.0+ 4.0+ no 1.0+
Remarks

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 the start or end of any line within the string to only the start or end of the entire string.

You cannot change this property directly.

Availability

JavaScript 1.2|JScript 5.5|ECMAScript v3

prototype : Object
Reference to the RegExp prototype object.
Show Details 4.0+ 1.0+ 4.0+ 7.0+ 1.0+
Availability

JavaScript 1.2|JScript 3.0|ECMAScript v3

rightContext : String
String containing the characters following the most recently found regular expression match.
Show Details 5.5+ 1.0+ 4.0+ no no
Availability

JavaScript 1.2|JScript 5.5|ECMAScript v3

source : String
A read-only property that contains the text of the pattern, excluding the forward slashes.
Show Details 4.0+ 1.0+ 4.0+ 7.0+ 1.0+
Remarks

source is a property of an individual regular expression object.

You cannot change this property directly.

Availability

JavaScript 1.2|JScript 3.0|ECMAScript v3

Functions

Method Action IE Mozilla Netscape Opera Safari
compile(Object expr, Object flags) : RegExp
Creates a new regular expression by compiling the specified regular expression with the specified flags.
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Parameters

Object expr The regular expression to be compiled.
Object flags The flags to be compiled with the regular expression.

Returns

RegExp
Availability

JavaScript 1.2|JScript 3.0|ECMAScript v3

exec(String regexp, String str) : Array
Executes the search for a match in a specified string. Returns a result array.
Show Details 4.0+ 1.0+ 4.0+ 7.0+ 1.0+

Parameters

String regexp The name of the regular expression. It can be a variable name or a literal.
String str The string against which to match the regular expression.

Returns

Array

Using exec

In the following example, the user enters a name and the script executes a match against the input. It then cycles through the array to see if other names match the user's name.

This script assumes that first names of registered party attendees are preloaded into the array A, perhaps by gathering them from a party database.

                        A = ["Frank", "Emily", "Jane", "Harry", "Nick", "Beth", "Rick",
                        "Terrence", "Carol", "Ann", "Terry", "Frank", "Alice", "Rick",
                        "Bill", "Tom", "Fiona", "Jane", "William", "Joan", "Beth"]
                        
                        function lookup() {
                        firstName = /\w+/i();
                        if (!firstName)
                        window.alert (RegExp.input + " isn't a name!");
                        else {
                        count = 0;
                        for (i=0; i<A.length; i++)
                        if (firstName[0].toLowerCase() == A[i].toLowerCase()) count++;
                        if (count ==1)
                        midstring = " other has ";
                        else
                        midstring = " others have ";
                        window.alert ("Thanks, " + count + midstring + "the same name!")
                        }
                        }
                        
                        <p>Enter your first name and then press Enter.</p>
                        
                        <FORM> <INPUT TYPE:"TEXT" NAME="FirstName" onChange="lookup(this);"> </ FORM>
                        
                        
Remarks

As shown in the syntax description, a regular expression's exec method can be called either directly, (with regexp.exec(str)) or indirectly (with regexp(str)).

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

If the match succeeds, the exec method returns an array and updates properties of the regular expression object. If the match fails, the exec method returns null.

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
                        myRe=/d(b+)(d)/ig;
                        myArray = myRe.exec("cdbBdbsbz");
                        

The following table shows the results for this script:

Object Property/Index Description Example
myArray   The content of myArray. ["dbBd", "bB", "d"]
index The 0-based index of the match in the string. 1
input The original string. cdbBdbsbz
[0] The last matched characters dbBd
[1], ...[n] The parenthesized substring matches, if any. The number of possible parenthesized substrings is unlimited. [1] = bB
[2] = d
myRe lastIndex The index at which to start the next match. 5
ignoreCase Indicates if the "i" flag was used to ignore case. true
global Indicates if the "g" flag was used for a global match. true
multiline Indicates if the "m" flag was used for a global match. false
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. For example, assume you have this script:

                        myRe=/ab*/g;
                        str = "abbcdefabh";
                        myArray = myRe.exec(str);
                        document.writeln("Found " + myArray[0] +
                        ". Next match starts at " + myRe.lastIndex)
                        mySecondArray = myRe.exec(str);
                        document.writeln("Found " + mySecondArray[0] +
                        ". Next match starts at " + myRe.lastIndex)
                        

This script displays the following text:

Found abb. Next match starts at 3
                        Found ab. Next match starts at 9. 
Throws
Throws TypeError if the method is invoked on an object that is not a RegExp.
See Also

RegExp.lastIndex|RegExp.test|String.match|String.replace|String.search

Availability

JavaScript 1.2|JScript 3.0|ECMAScript v3

test(String regexp, String str) : Boolean
Executes the search for a match between a regular expression and a specified string.
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Parameters

String regexp The name of the regular expression. It can be a variable name or a literal.
String str The string against which to match the regular expression.

Returns

Boolean

Using test

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);
                        }
Remarks

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).

Throws
Throws TypeError if the method is invoked on an object that is not a RegExp.
See Also

RegExp.exec|RegExp.lastIndex|String.match|String.replace|String.substring

Availability

JavaScript 1.2|JScript 3.0|ECMAScript v3

toSource() : String
Returns a string representing the source code of the object.
Show Details 4.0+ 1.0+ 4.0+ no no

Returns

String
Remarks

The toSource method returns the following values:

  • For the built-in RegExp object, toSource returns the following string indicating that the source code is not available/
function Boolean() {
                        [native code]
                        }
                        
  • For instances of RegExp, toSource returns a string representing the source code.

This method is usually called internally by JavaScript and not explicitly in code.

See Also

Object.toSource

Availability

toString() : String
Returns a string representing the specified object.
Show Details 4.0+ 1.0+ 4.0+ 7.0+ 1.0+

Returns

String

Using toString

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

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

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 object.

Throws
Throws TypeError if the method is invoked on an object that is not a RegExp.
See Also

Object.toString

Availability

JavaScript 1.2|JScript 3.0|ECMAScript v3

Using the replace method

The following script uses the replace method to switch the words in the string. 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");
         document.write(newstr);

This displays "Smith, John".

Using input

In the following example, RegExp.input is set by the Change event. In the getInfo function, the exec method uses the value of RegExp.input as its argument.

         function getInfo() {
         var re = /(\w+)\s(\d+)/;
         var m = re.exec();
         window.alert(m[0] + ", your age is " + m[2]);
         }
         
         Enter your first name and your age, and then press Enter.
         
         <form>
         <input type="text" name="NameAge" onchange="getInfo(this);"/>
         </form>

Remarks

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:

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

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 character.

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-c]. 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. Equivalent to [0-9].

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

\D

Matches any non-digit character. 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. Equivalent to [ \f\n\r\t\u00A0\u2028\u2029].

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

\S

Matches a single character other than white space. Equivalent to [^ \f\n\r\t\u00A0\u2028\u2029].

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

\t

Matches a tab.

\v

Matches a vertical tab.

\w

Matches any alphanumeric character 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 non-word character. 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 code 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.

A separate predefined RegExp object is available in each window; that is, each separate thread of JavaScript execution gets its own RegExp object. Because each script runs to completion without interruption in a thread, this assures that different scripts do not overwrite values of the RegExp object.

Availability

JavaScript 1.2|JScript 3.0|ECMAScript v3

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