Regular Expressions and RegExp Object


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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 multiline 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

^p

It 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.p

It 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

Alphanumeric

Itself

2

\0

The NUL character (\u0000)

3

\t

Tab (\u0009

4

\n

Newline (\u000A)

5

\v

Vertical tab (\u000B)

6

\f

Form feed (\u000C)

7

\r

Carriage return (\u000D)

8

\xnn

The Latin character specified by the hexadecimal number nn; for example, \x0A is the same as \n

9

\uxxxx

The Unicode character specified by the hexadecimal number xxxx; for example, \u0009 is the same as \t

10

\cX

The 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

\s

a whitespace character (space, tab, newline)

3

\S

non-whitespace character

4

\d

a digit (0-9)

5

\D

a non-digit

6

\w

a word character (a-z, A-Z, 0-9, _)

7

\W

a 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

i

Perform case-insensitive matching.

2

m

Specifies 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

g

Performs 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 constructor

Specifies the function that creates an object's prototype.

2 global

Specifies if the "g" modifier is set.

3 ignoreCase

Specifies if the "i" modifier is set.

4 lastIndex

The index at which to start the next match.

5 multiline

Specifies if the "m" modifier is set.

6 source

The 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.



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