# RegExp : Object
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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. | Show Details | 4.0+ | 1.0+ | 4.0+ | 7.0+ | 1.0+

### 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. | Show Details | 4.0+ | 1.0+ | 4.0+ | 7.0+ | no

Availability

  JavaScript 1.2|JScript 3.0

constructor : Object Specifies the function that creates the RegExp prototype. | Show Details | 4.0+ | 1.0+ | 4.0+ | 7.0+ | 1.0+

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. | Show Details | 4.0+ | 1.0+ | 4.0+ | 7.0+ | 1.0+

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. | Show Details | 4.0+ | 1.0+ | 4.0+ | 7.0+ | 1.0+

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. | Show Details | 4.0+ | 1.0+ | 4.0+ | 7.0+ | no

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 `String` `search` 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. | 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

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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