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RegExp

A regular expression object contains the pattern of a regular expression. It has properties and methods for using that regular expression to find and replace matches in strings.

In addition to the properties of an individual regular expression object that you create using the RegExp constructor function, the predefined RegExp object has static properties that are set whenever any regular expression is used.

Core object

Implemented in

JavaScript 1.2, NES 3.0

JavaScript 1.3: added toSource method

Created by

A literal text format or the RegExp constructor function.

The literal format is used as follows:

/pattern/flags
The constructor function is used as follows:

new RegExp("pattern"[, "flags"])

Parameters

pattern

The text of the regular expression.

flags

If specified, flags can have one of the following values:

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")

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:

re = new RegExp("\\w+")
re = /\w+/
The following table provides a complete list and description of the special characters that can be used in regular expressions.

Table 1.3 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 or line.

For example, /^A/ does not match the 'A' in "an A," but does match it in "An A."

$

Matches end of input or line.

For example, /t$/ does not match the 't' in "eater", but does match it in "eat"

*

Matches the preceding character 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 character 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 character 0 or 1 time.

For example, /e?le?/ matches the 'el' in "angel" and the 'le' in "angle."

.

(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.

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|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 character.

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 character.

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 character.

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 control character. 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\v].

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

\S

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

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.

Note: If the number of left parentheses is less than the number specified in \n, the \n is taken as an octal escape as described in the next row.

\ooctal
\xhex

Where \ooctal is an octal escape value or \xhex is a hexadecimal escape value. Allows you to embed ASCII codes into regular expressions.

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. Once you have a defined regular expression, and if the regular expression is used throughout the script and may change, you can use the compile method to compile a new regular expression for efficient reuse.

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.

The predefined RegExp object contains the static properties input, multiline, lastMatch, lastParen, leftContext, rightContext, and $1 through $9. The input and multiline properties can be preset. The values for the other static properties are set after execution of the exec and test methods of an individual regular expression object, and after execution of the match and replace methods of String.

Property Summary

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.

Property Description
$1, ..., $9

Parenthesized substring matches, if any.

$_

See input.

$*

See multiline.

$&

See lastMatch.

$+

See lastParen.

$`

See leftContext.

$'

See rightContext.

constructor

Specifies the function that creates an object's prototype.

global

Whether or not to test the regular expression against all possible matches in a string, or only against the first.

ignoreCase

Whether or not to ignore case while attempting a match in a string.

input

The string against which a regular expression is matched.

lastIndex

The index at which to start the next match.

lastMatch

The last matched characters.

lastParen

The last parenthesized substring match, if any.

leftContext

The substring preceding the most recent match.

multiline

Whether or not to search in strings across multiple lines.

prototype

Allows the addition of properties to all objects.

rightContext

The substring following the most recent match.

source

The text of the pattern.

Method Summary

Method Description
compile

Compiles a regular expression object.

exec

Executes a search for a match in its string parameter.

test

Tests for a match in its string parameter.

toSource

Returns an object literal representing the specified object; you can use this value to create a new object. Overrides the Object.toSource method.

toString

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

valueOf

Returns the primitive value of the specified object. Overrides the Object.valueOf method.

In addition, this object inherits the watch and unwatch methods from Object.

Examples

Example 1. The following script uses the replace method to switch the words in the string. For the replacement text, the script uses the values of the $1 and $2 properties of the global RegExp object. Note that the RegExp object name is not be prepended to the $ properties when they are passed as the second argument to the replace method.

<SCRIPT LANGUAGE="JavaScript1.2">
re = /(\w+)\s(\w+)/;
str = "John Smith";
newstr=str.replace(re, "$2, $1");
document.write(newstr)
</SCRIPT>
This displays "Smith, John".

Example 2. 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. Note that RegExp is prepended to the $ properties.

<HTML>
<SCRIPT LANGUAGE="JavaScript1.2">
function getInfo() {
   re = /(\w+)\s(\d+)/;
   re.exec();
   window.alert(RegExp.$1 + ", your age is " + RegExp.$2);
}
</SCRIPT>
Enter your first name and your age, and then press Enter.
<FORM>
<INPUT TYPE:"TEXT" NAME="NameAge" onChange="getInfo(this);">
</FORM>
</HTML>

$1, ..., $9

Properties that contain parenthesized substring matches, if any.

Property of

RegExp

Static, Read-only

Implemented in

JavaScript 1.2, NES 3.0

Description

Because input is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.input.

The number of possible parenthesized substrings is unlimited, but the predefined RegExp object can only hold the last nine. You can access all parenthesized substrings through the returned array's indexes.

These properties can be used in the replacement text for the String.replace method. When used this way, do not prepend them with RegExp. The example below illustrates this. When parentheses are not included in the regular expression, the script interprets $n's literally (where n is a positive integer).

Examples

The following script uses the replace method to switch the words in the string. For the replacement text, the script uses the values of the $1 and $2 properties of the global RegExp object. Note that the RegExp object name is not be prepended to the $ properties when they are passed as the second argument to the replace method.

<SCRIPT LANGUAGE="JavaScript1.2">
re = /(\w+)\s(\w+)/;
str = "John Smith";
newstr=str.replace(re, "$2, $1");
document.write(newstr)
</SCRIPT>
This displays "Smith, John".


$_

See input.


$*

See multiline.


$&

See lastMatch.


$+

See lastParen.


$`

See leftContext.


$'

See rightContext.


compile

Compiles a regular expression object during execution of a script.

Method of

RegExp

Implemented in

JavaScript 1.2, NES 3.0

Syntax

regexp.compile(pattern[, flags])

Parameters

regexp

The name of the regular expression. It can be a variable name or a literal.

pattern

A string containing the text of the regular expression.

flags

If specified, flags can have one of the following values:

Description

Use the compile method to compile a regular expression created with the RegExp constructor function. This forces compilation of the regular expression once only which means the regular expression isn't compiled each time it is encountered. Use the compile method when you know the regular expression will remain constant (after getting its pattern) and will be used repeatedly throughout the script.

You can also use the compile method to change the regular expression during execution. For example, if the regular expression changes, you can use the compile method to recompile the object for more efficient repeated use.

Calling this method changes the value of the regular expression's source, global, and ignoreCase properties.


constructor

Specifies the function that creates an object's prototype. Note that the value of this property is a reference to the function itself, not a string containing the function's name.

Property of

RegExp

Implemented in

JavaScript 1.1, NES 2.0

ECMA version

ECMA-262

Description

See Object.constructor.


exec

Executes the search for a match in a specified string. Returns a result array.

Method of

RegExp

Implemented in

JavaScript 1.2, NES 3.0

Syntax

regexp.exec([str])
regexp([str])

Parameters

regexp

The name of the regular expression. It can be a variable name or a literal.

str

The string against which to match the regular expression. If omitted, the value of RegExp.input is used.

Description

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 String search method.

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

Consider the following example:

<SCRIPT LANGUAGE="JavaScript1.2">
//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");
</SCRIPT>
The following table shows the results for this script:

Object Property/Index Description Example
myArray

The contents 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

source

The text of the pattern

d(b+)(d)

RegExp
lastMatch
$&

The last matched characters

dbBd

leftContext
$\Q

The substring preceding the most recent match

c

rightContext
$'

The substring following the most recent match

bsbz

$1, ...$9

The parenthesized substring matches, if any. The number of possible parenthesized substrings is unlimited, but RegExp can only hold the last nine.

$1 = bB 
$2 = d

lastParen  
$+

The last parenthesized substring match, if any.

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:

<SCRIPT LANGUAGE="JavaScript1.2">
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)
</SCRIPT>
This script displays the following text:

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

Examples

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.

<HTML>
<SCRIPT LANGUAGE="JavaScript1.2">
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!")
   }
}
</SCRIPT>
Enter your first name and then press Enter.
<FORM> <INPUT TYPE:"TEXT" NAME="FirstName" onChange="lookup(this);"> </FORM>
</HTML>

global

Whether or not the "g" flag is used with the regular expression.

Property of

RegExp

Read-only

Implemented in

JavaScript 1.2, NES 3.0

Description

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. However, calling the compile method changes the value of this property.


ignoreCase

Whether or not the "i" flag is used with the regular expression.

Property of

RegExp

Read-only

Implemented in

JavaScript 1.2, NES 3.0

Description

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. However, calling the compile method changes the value of this property.


input

The string against which a regular expression is matched. $_ is another name for the same property.

Property of

RegExp

Static

Implemented in

JavaScript 1.2, NES 3.0

Description

Because input is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.input.

If no string argument is provided to a regular expression's exec or test methods, and if RegExp.input has a value, its value is used as the argument to that method.

The script or the browser can preset the input property. If preset and if no string argument is explicitly provided, the value of input is used as the string argument to the exec or test methods of the regular expression object. input is set by the browser in the following cases:

The value of the input property is cleared after the event handler completes.


lastIndex

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

Property of

RegExp

Implemented in

JavaScript 1.2, NES 3.0

Description

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:

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.


lastMatch

The last matched characters. $& is another name for the same property.

Property of

RegExp

Static, Read-only

Implemented in

JavaScript 1.2, NES 3.0

Description

Because lastMatch is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.lastMatch.


lastParen

The last parenthesized substring match, if any. $+ is another name for the same property.

Property of

RegExp

Static, Read-only

Implemented in

JavaScript 1.2, NES 3.0

Description

Because lastParen is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.lastParen.


leftContext

The substring preceding the most recent match. $` is another name for the same property.

Property of

RegExp

Static, Read-only

Implemented in

JavaScript 1.2, NES 3.0

Description

Because leftContext is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.leftContext.


multiline

Reflects whether or not to search in strings across multiple lines. $* is another name for the same property.

Property of

RegExp

Static

Implemented in

JavaScript 1.2, NES 3.0

Description

Because multiline is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.multiline.

The value of multiline is true if multiple lines are searched, false if searches must stop at line breaks.

The script or the browser can preset the multiline property. When an event handler is called for a TEXTAREA form element, the browser sets multiline to true. multiline is cleared after the event handler completes. This means that, if you've preset multiline to true, it is reset to false after the execution of any event handler.


prototype

Represents the prototype for this class. You can use the prototype to add properties or methods to all instances of a class. For information on prototypes, see Function.prototype.

Property of

RegExp

Implemented in

JavaScript 1.1, NES 2.0

ECMA version

ECMA-262


rightContext

The substring following the most recent match. $' is another name for the same property.

Property of

RegExp

Static, Read-only

Implemented in

JavaScript 1.2, NES 3.0

Description

Because rightContext is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.rightContext.


source

A read-only property that contains the text of the pattern, excluding the forward slashes and "g" or "i" flags.

Property of

RegExp

Read-only

Implemented in

JavaScript 1.2, NES 3.0

Description

source is a property of an individual regular expression object.

You cannot change this property directly. However, calling the compile method changes the value of this property.


test

Executes the search for a match between a regular expression and a specified string. Returns true or false.

Method of

RegExp

Implemented in

JavaScript 1.2, NES 3.0

Syntax

regexp.test([str])

Parameters

regexp

The name of the regular expression. It can be a variable name or a literal.

str

The string against which to match the regular expression. If omitted, the value of RegExp.input is used.

Description

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).

Example

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);
}

toSource

Returns a string representing the source code of the object.

Method of

RegExp

Implemented in

JavaScript 1.3

Syntax

toSource()

Parameters

None

Description

The toSource method returns the following values:

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

See also

Object.toSource


toString

Returns a string representing the specified object.

Method of

RegExp

Implemented in

JavaScript 1.1, NES 2.0

ECMA version

ECMA-262

Syntax

toString()

Parameters

None.

Description

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.

Examples

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

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

See also

Object.toString


valueOf

Returns the primitive value of a RegExp object.

Method of

RegExp

Implemented in

JavaScript 1.1

ECMA version

ECMA-262

Syntax

valueOf()

Parameters

None

Description

The valueOf method of RegExp returns the primitive value of a RegExp object as a string data type. This value is equivalent to RegExp.toString.

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

Examples

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

See also

RegExp.toString, Object.valueOf


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Last Updated: 11/16/98 12:57:08

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