JavaScript RegExp (Regular Expression)

Last Updated : 03 Dec, 2024

A regular expression is a special sequence of characters that defines a search pattern, typically used for pattern matching within text. It’s often used for tasks such as validating email addresses, phone numbers, or checking if a string contains certain patterns (like dates, specific words, etc.).

In JavaScript, RegExp is an object that is used to create regular expressions, which are patterns used to match character combinations in strings.

JavaScript

// The /i flag makes the pattern case-insensitive const patt = /Geeks/i;  const s1 = "geeksforgeeks"; const s2 = "forgeeks";  console.log(patt.test(s1)); console.log(patt.test(s2));

Output

true true

Creating a RegExp in JavaScript

There are two primary ways to create a RegExp in JavaScript

1. Using the RegExp Constructor

The RegExp constructor is useful when you need to create a regular expression dynamically, such as when the pattern or flags might change based on user input or other conditions.

JavaScript

let pattern = "hello";  // Pattern to match let flags = "i";  // Case-insensitive flag let regex = new RegExp(pattern, flags);  let s = "Hello world";  console.log(regex.test(s));

Output

true

Here, we dynamically create a RegExp object using the RegExp() constructor.
The pattern “hello” will match the string “Hello” because of the ‘i’ flag, which makes the search case-insensitive.
regex.test(s) returns true because “Hello” matches the pattern “hello” in a case-insensitive manner.

2. Using Regular Expression Literal

This is the most common and simple way to create a regular expression in JavaScript. It’s especially useful when the pattern is static (doesn’t change dynamically).

JavaScript

let regex = /hello/i; let s = "Hello world"; console.log(regex.test(s));

Output

true

In this example, we define the regular expression directly using literal notation.
/hello/i means we’re looking for the word “hello”, and the ‘i’ flag ensures the search is case-insensitive.
regex.test(s) returns true because “Hello” matches the pattern “hello” case-insensitively.

Regular Expression Modifiers can be used to perform multiline searches which can also be set to case-insensitive matching:

Expressions	Descriptions
g	Find the character globally
i	Find a character with case-insensitive matching
m	Find multiline matching

Regular Expression Brackets can Find characters in a specified range

Expressions	Description
[abc]	Find any of the characters inside the brackets
[^abc]	Find any character, not inside the brackets
[0-9]	Find any of the digits between the brackets 0 to 9
[^0-9]	Find any digit not in between the brackets
(x \| y)	Find any of the alternatives between x or y separated with \|

Regular Expression Metacharacters are characters with a special meaning:

Metacharacter	Description
\.	Search single characters, except line terminator or newline.
\w	Find the word character i.e. characters from a to z, A to Z, 0 to 9
\d	Find a digit
\D	Search non-digit characters i.e all the characters except digits
\s	Find a whitespace character
\S	Find the non-whitespace characters.
\b	Find a match at the beginning or at the end of a word
\B	Find a match that is not present at the beginning or end of a word.
\0	Find the NULL character.
\n	Find the newline character.
\f	Find the form feed character
\r	Find the carriage return character
\t	Find the tab character
\v	Find the vertical tab character
\uxxxx	Find the Unicode character specified by the hexadecimal number xxxxx

Regular Expression Quantifiers are used to define quantitiesoccurrence

Quantifier	Description
n+	Match any string that contains at least one n
n*	Match any string that contains zero or more occurrences of n
n?	Match any string that contains zero or one occurrence of n
m{X}	Find the match of any string that contains a sequence of m, X times
m{X, Y}	Find the match of any string that contains a sequence of m, X to Y times
m{X,}	Find the match of any string that contains a sequence of m, at least X times
m$	Find the match of any string which contains m at the end of it
^m	Find the match of any string which contains m at the beginning of it
?!m	Find the match of any string which is not followed by a specific string m.

Regular Expression Object Properties:

Property	Description
constructor	Return the function that created the RegExp object’s prototype
global	Specify whether the “g” modifier is set or not
ignorecase	Specify whether the “i” modifier is set or not
lastindex	Specify the index at which to start the next match
multiline	Specify whether the “m” modifier is set or not
source	Return the text of RegExp pattern

Regular Expression Object Methods:

Method	Description
compile()	Used to compile the regular expression while executing of script
exec()	Used to test for the match in a string.
test()	Used to test for a match in a string
toString()	Return the string value of the regular expression

Use Cases of Regular Expressions

1. Validating Email Addresses

Regular expressions are frequently used for validating emails. Here’s an example that matches most email formats

JavaScript

let regex = /^[a-zA-Z0-9._-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,6}$/; console.log(regex.test("[email protected]"));

Output

true

2. Extracting Specific Data from a String

You can use regular expressions to extract specific data from a string, like extracting all numbers from a text.

JavaScript

let regex = /\d+/g; let res = "There are 123 apples and 456 oranges".match(regex); console.log(res);

Output

[ '123', '456' ]

3. Replacing Substrings

Regular expressions are often used in replacing certain substrings in text.

JavaScript

let regex = /foo/g; let res = "foo bar foo".replace(regex, "baz"); console.log(res);

Output

baz bar baz

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Article Tags :

JavaScript RegExp (Regular Expression)

Creating a RegExp in JavaScript

1. Using the RegExp Constructor

2. Using Regular Expression Literal

Use Cases of Regular Expressions

1. Validating Email Addresses

2. Extracting Specific Data from a String

3. Replacing Substrings

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