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.
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Core object
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Implemented in
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JavaScript 1.2, NES 3.0
JavaScript 1.3: added toSource method.
JavaScript 1.5, NES 6.0: added m flag, non-greedy modifier, non-capturing parentheses, lookahead assertions. ECMA 262, Edition 3
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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
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pattern
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The text of the regular expression.
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flags
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If specified, flags can have any combination of the following values:
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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 1.1 Special characters in regular expressions.
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Character
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Meaning
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\
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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*'.
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^
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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."
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$
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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".
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*
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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".
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+
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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".
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?
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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.
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.
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(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'.
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(x)
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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.
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(?:x)
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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.
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x(?=y)
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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.
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x(?!y)
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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.
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x|y
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Matches either 'x' or 'y'.
For example, /green|red/ matches 'green' in "green apple" and 'red' in "red apple."
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{n}
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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."
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{n,}
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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."
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{n,m}
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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.
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[xyz]
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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".
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[^xyz]
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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."
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[\b]
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Matches a backspace. (Not to be confused with \b.)
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\b
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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."
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\B
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Matches a non-word boundary.
For example, /\w\Bn/ matches 'on' in "noonday", and /y\B\w/ matches 'ye' in "possibly yesterday."
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\cX
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Where X is a letter from A - Z. Matches a control character in a string.
For example, /\cM/ matches control-M in a string.
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\d
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Matches a digit character. Equivalent to [0-9].
For example, /\d/ or /[0-9]/ matches '2' in "B2 is the suite number."
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\D
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Matches any non-digit character. Equivalent to [^0-9].
For example, /\D/ or /[^0-9]/ matches 'B' in "B2 is the suite number."
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\f
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Matches a form-feed.
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Matches a linefeed.
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Matches a carriage return.
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\s
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Matches a single white space character, including space, tab, form feed, line feed. Equivalent to [ \f
\u00A0\u2028\u2029].
For example, /\s\w*/ matches ' bar' in "foo bar."
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\S
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Matches a single character other than white space. Equivalent to
[^ \f
\u00A0\u2028\u2029].
For example, /\S/\w* matches 'foo' in "foo bar."
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Matches a tab.
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\v
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Matches a vertical tab.
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\w
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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."
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\W
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Matches any non-word character. Equivalent to [^A-Za-z0-9_].
For example, /\W/ or /[^-Za-z0-9_]/ matches '%' in "50%."
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\n
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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/ matches 'apple, orange', in "apple, orange, cherry, peach." A more complete example follows this table.
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Matches a NUL character. Do not follow this with another digit.
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\xhh
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Matches the character with the code hh (two hexadecimal digits)
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\uhhhh
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Matches the character with code hhhh (four hexadecimal digits).
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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.
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.
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Property
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Description
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constructor
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Specifies the function that creates an object's prototype.
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global
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Whether to test the regular expression against all possible matches in a string, or only against the first. As of JavaScript 1.5, a property of a RegExp instance, not the RegExp object.
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ignoreCase
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Whether to ignore case while attempting a match in a string. As of JavaScript 1.5, a property of a RegExp instance, not the RegExp object.
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lastIndex
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The index at which to start the next match. As of JavaScript 1.5, a property of a RegExp instance, not the RegExp object.
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multiline
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Whether or not to search in strings across multiple lines. As of JavaScript 1.5, a property of a RegExp instance, not the RegExp object.
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prototype
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Allows the addition of properties to all objects.
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source
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The text of the pattern. As of JavaScript 1.5, a property of a RegExp instance, not the RegExp object.
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Method Summary
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Method
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Description
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exec
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Executes a search for a match in its string parameter.
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test
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Tests for a match in its string parameter.
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toSource
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Returns an object literal representing the specified object; you can use this value to create a new object. Overrides the Object.toSource method.
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toString
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Returns a string representing the specified object. Overrides the Object.toString method.
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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. In the replacement text, the script uses "$1" and "$2" to indicate the results of the corresponding matching parentheses in the regular expression pattern.
<SCRIPT>
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.
<HTML>
<SCRIPT>
function getInfo() {
re = /(\w+)\s(\d+)/;
var m = re.exec();
window.alert(m[] + ", your age is " + m[2]);
}
</SCRIPT>
Enter your first name and your age, and then press Enter.
<FORM>
<INPUT TYPE:"TEXT" NAME="NameAge" onChange="getInfo(this);">
</FORM>
</HTML>
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.
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Property of
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RegExp
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Implemented in
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JavaScript 1.1, NES 2.0
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ECMA version
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ECMA-262
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Description
See Object.constructor.
exec
Executes the search for a match in a specified string. Returns a result array.
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Method of
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RegExp
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Implemented in
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JavaScript 1.2, NES 3.0
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ECMA version
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ECMA 262, Edition 3 (first syntax only)
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Syntax
regexp.exec([str])
regexp([str])
Parameters
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regexp
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The name of the regular expression. It can be a variable name or a literal.
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str
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The string against which to match the regular expression.
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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. 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:
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Object
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Property/Index
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Description
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Example
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myArray
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The contents of myArray.
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["dbBd", "bB", "d"]
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index
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The 0-based index of the match in the string.
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1
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input
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The original string.
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cdbBdbsbz
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[0]
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The last matched characters.
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dbBd
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[1], ...[n]
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The parenthesized substring matches, if any. The number of possible parenthesized substrings is unlimited.
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[1] = bB
[2] = d
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myRe
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lastIndex
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The index at which to start the next match.
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5
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ignoreCase
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Indicates if the "i" flag was used to ignore case.
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true
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global
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Indicates if the "g" flag was used for a global match.
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true
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multiline
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Indicates if the "m" flag was used for a global match.
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false
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source
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The text of the pattern.
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d(b+)(d)
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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.
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Property of
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RegExp instances
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Read-only
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Implemented in
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JavaScript 1.2, NES 3.0
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ECMA version
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ECMA 262, Edition 3
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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.
ignoreCase
Whether or not the "i" flag is used with the regular expression.
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Property of
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RegExp instances
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Read-only
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Implemented in
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JavaScript 1.2, NES 3.0
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ECMA version
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ECMA 262, Edition 3
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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.
lastIndex
A read/write integer property that specifies the index at which to start the next match.
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Property of
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RegExp instances
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Implemented in
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JavaScript 1.2, NES 3.0
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ECMA version
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ECMA 262, Edition 3
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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:
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:
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re = /(hi)?/g
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Matches the empty string.
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re("hi")
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Returns ["hi", "hi"] with lastIndex equal to 2.
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re("hi")
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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.
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multiline
Reflects whether or not to search in strings across multiple lines.
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Property of
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RegExp instances
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Static
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Implemented in
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JavaScript 1.2, NES 3.0
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ECMA version
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ECMA 262, Edition 3
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Description
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.
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.
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Property of
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RegExp
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Implemented in
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JavaScript 1.1, NES 2.0
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ECMA version
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ECMA-262
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source
A read-only property that contains the text of the pattern, excluding the forward slashes.
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Property of
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RegExp instances
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Read-only
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Implemented in
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JavaScript 1.2, NES 3.0
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ECMA version
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ECMA 262, Edition 3
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Description
source is a property of an individual regular expression object.
You cannot change this property directly.
test
Executes the search for a match between a regular expression and a specified string. Returns true or false.
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Method of
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RegExp
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Implemented in
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JavaScript 1.2, NES 3.0
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ECMA version
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ECMA 262, Edition 3
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Syntax
regexp.test([str])
Parameters
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regexp
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The name of the regular expression. It can be a variable name or a literal.
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str
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The string against which to match the regular expression.
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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.
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Method of
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RegExp
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Implemented in
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JavaScript 1.3
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Syntax
toSource()
Parameters
None
Description
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
toString
Returns a string representing the specified object.
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Method of
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RegExp
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Implemented in
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JavaScript 1.1, NES 2.0
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ECMA version
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ECMA 262, Edition 3
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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
