A regular expression is an object that describes a pattern of characters.

The JavaScript RegExp class represents regular expressions, and both String and RegExp define methods that use regular expressions to perform powerful pattern-matching and search-and-replace functions on text.

Syntax

A regular expression could be defined with the RegExp () constructor, as follows −

var pattern = new RegExp(pattern, attributes);

or simply

var pattern = /pattern/attributes;

Here is the description of the parameters −

·        pattern − A string that specifies the pattern of the regular expression or another regular expression.

·        attributes − An optional string containing any of the “g”, “i”, and “m” attributes that specify global, case-insensitive, and multi-line matches, respectively.

Brackets

Brackets ([]) have a special meaning when used in the context of regular expressions. They are used to find a range of characters.

Sr.No.	Expression & Description
1	[…]Any one character between the brackets.
2	[^…]Any one character not between the brackets.
3	[0-9]It matches any decimal digit from 0 through 9.
4	[a-z]It matches any character from lowercase a through lowercase z.
5	[A-Z]It matches any character from uppercase A through uppercase Z.
6	[a-Z]It matches any character from lowercase a through uppercase Z.

The ranges shown above are general; you could also use the range [0-3] to match any decimal digit ranging from 0 through 3, or the range [b-v] to match any lowercase character ranging from b through v.

Quantifiers

The frequency or position of bracketed character sequences and single characters can be denoted by a special character. Each special character has a specific connotation. The +, *, ?, and $ flags all follow a character sequence.

Sr.No.	Expression & Description
1	p+It matches any string containing one or more p’s.
2	p*It matches any string containing zero or more p’s.
3	p?It matches any string containing at most one p.
4	p{N}It matches any string containing a sequence of N p’s
5	p{2,3}It matches any string containing a sequence of two or three p’s.
6	p{2, }It matches any string containing a sequence of at least two p’s.
7	p$It matches any string with p at the end of it.
8	^pIt matches any string with p at the beginning of it.

Examples

Following examples explain more about matching characters.

Sr.No.	Expression & Description
1	[^a-zA-Z]It matches any string not containing any of the characters ranging from a through z and A through Z.
2	p.pIt matches any string containing p, followed by any character, in turn followed by another p.
3	^.{2}$It matches any string containing exactly two characters.
4	<b>(.*)</b>It matches any string enclosed within <b> and </b>.
5	p(hp)*It matches any string containing a p followed by zero or more instances of the sequence hp.

Literal characters

Sr.No.	Character & Description
1	AlphanumericItself
2	\0The NUL character (\u0000)
3	\tTab (\u0009
4	\nNewline (\u000A)
5	\vVertical tab (\u000B)
6	\fForm feed (\u000C)
7	\rCarriage return (\u000D)
8	\xnnThe Latin character specified by the hexadecimal number nn; for example, \x0A is the same as \n
9	\uxxxxThe Unicode character specified by the hexadecimal number xxxx; for example, \u0009 is the same as \t
10	\cXThe control character ^X; for example, \cJ is equivalent to the newline character \n

Metacharacters

A metacharacter is simply an alphabetical character preceded by a backslash that acts to give the combination a special meaning.

For instance, you can search for a large sum of money using the ‘\d’ metacharacter: /([\d]+)000/, Here \d will search for any string of numerical character.

The following table lists a set of metacharacters which can be used in PERL Style Regular Expressions.

Sr.No.	Character & Description
1	.a single character
2	\sa whitespace character (space, tab, newline)
3	\Snon-whitespace character
4	\da digit (0-9)
5	\Da non-digit
6	\wa word character (a-z, A-Z, 0-9, _)
7	\Wa non-word character
8	[\b]a literal backspace (special case).
9	[aeiou]matches a single character in the given set
10	[^aeiou]matches a single character outside the given set
11	(foo|bar|baz)matches any of the alternatives specified

Modifiers

Several modifiers are available that can simplify the way you work with regexps, like case sensitivity, searching in multiple lines, etc.

Sr.No.	Modifier & Description
1	iPerform case-insensitive matching.
2	mSpecifies that if the string has newline or carriage return characters, the ^ and $ operators will now match against a newline boundary, instead of a string boundary
3	gPerforms a global matchthat is, find all matches rather than stopping after the first match.

RegExp Properties

Here is a list of the properties associated with RegExp and their description.

Sr.No.	Property & Description
1	constructorSpecifies the function that creates an object’s prototype.
2	globalSpecifies if the “g” modifier is set.
3	ignoreCaseSpecifies if the “i” modifier is set.
4	lastIndexThe index at which to start the next match.
5	multilineSpecifies if the “m” modifier is set.
6	sourceThe text of the pattern.

In the following sections, we will have a few examples to demonstrate the usage of RegExp properties.

RegExp Methods

Here is a list of the methods associated with RegExp along with their description.

Sr.No.	Method & Description
1	exec()Executes a search for a match in its string parameter.
2	test()Tests for a match in its string parameter.
3	toSource()Returns an object literal representing the specified object; you can use this value to create a new object.
4	toString()Returns a string representing the specified object.

In the following sections, we will have a few examples to demonstrate the usage of RegExp methods.