Regular Expression

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. Regular expression is a core object.

Note:

In JavaScript 1.5, some characters such as pipes (|) which were treated a characters in certain circumstances are now treated as regular expression symbols. See also http://developer.mozilla.org/en/docs/Core_JavaScript_1.5_Reference.

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

(Optional) If specified, flags can have one of the following 3 values:

G —global match
i—ignore case
gi—both global match and ignore case

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.

Character	Description
\	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.

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.

re = /(\w+)\s(\w+)/;
str = "John Smith";
newstr=str.replace(re, "$2, $1");
(newstr)

This displays:

"Smith, John".

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.

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