Regular Expressions - RegEx.md

Regular Expressions in JavaScript with ES6+

Table of Contents

Introduction to Regular Expressions Creating Regular Expressions Regular Expression Methods Regular Expression Syntax - Basic Characters - Quantifiers - Special Characters - Groups and Capturing - Flags Examples - Email Validation - URL Validation - Date Validation (YYYY-MM-DD)

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Introduction to Regular Expressions

Regular expressions (RegEx) are powerful patterns used to match character combinations in strings. They are a fundamental tool for text processing and validation in JavaScript. With ES6+, JavaScript introduced several enhancements to make working with regular expressions more efficient and expressive.

Creating Regular Expressions

Literal Syntax

Regular expressions can be created using literal syntax by enclosing the pattern in forward slashes (/).

const regex = /pattern/;

Constructor Syntax

Regular expressions can also be created using the RegExp constructor.

const regex = new RegExp('pattern');

ES6+ Enhancements

ES6 introduced template literals, which can be used to create dynamic regular expressions.

const dynamicPattern = 'hello';
const regex = new RegExp(`${dynamicPattern}`);

Regular Expression Methods

test() : The test() method tests for a match in a string. It returns true or false.

const regex = /hello/;
const result = regex.test('hello world'); // true

exec() : The exec() method executes a search for a match in a string. It returns an array of information or null if no match is found.

const regex = /hello/;
const result = regex.exec('hello world'); // ['hello']

match() : The match() method returns an array containing the matches in a string.

const regex = /hello/;
const result = 'hello world'.match(regex); // ['hello']

replace() : The replace() method replaces matches in a string with a specified value.

const regex = /hello/;
const result = 'hello world'.replace(regex, 'hi'); // 'hi world'

search() : The search() method searches for a match in a string. It returns the index of the first match or -1 if no match is found.

const regex = /hello/;
const result = 'hello world'.search(regex); // 0

split() : The split() method splits a string into an array of substrings using a regular expression.

const regex = /,/;
const result = 'apple,banana,cherry'.split(regex); // ['apple', 'banana', 'cherry']

Regular Expression Syntax

Basic Characters

Symbol	Meaning
.	Any character except newline
\d	Digit (0-9)
\D	Not a digit
\w	Word character (alphanumeric + underscore)
\W	Not a word character
\s	Whitespace character
\S	Not a whitespace character

Quantifiers

Symbol	Meaning
*	0 or more occurrences
+	1 or more occurrences
?	0 or 1 occurrence
{n}	Exactly n occurrences
{n,}	n or more occurrences
{n,m}	Between n and m occurrences

Special Characters

Symbol	Meaning
^	Start of string
$	End of string
\	Escape character
[]	Character set
[^]	Negated character set
|	Alternation (OR)
()	Capturing group

Groups and Capturing

Symbol	Meaning
()	Capturing group
(?:)	Non-capturing group
\1, \2	Backreferences to captured groups

Flags

Flag	Meaning
g	Global search (find all matches)
i	Case-insensitive search
m	Multi-line search

Examples

Email Validation

const emailRegex = /^[\w.-]+@[\w.-]+\.\w+$/;
const isValidEmail = emailRegex.test('[email protected]'); // true

URL Validation

const urlRegex = /^(https?:\/\/)?([\da-z\.-]+)\.([a-z\.]{2,6})([\/\w \.-]*)*\/?$/;
const isValidUrl = urlRegex.test('https://www.example.com'); // true

Date Validation (YYYY-MM-DD)

const dateRegex = /^\d{4}-\d{2}-\d{2}$/;
const isValidDate = dateRegex.test('2023-10-05'); // true

Tip

Use online tools like regex101.com to test and debug your regular expressions. Leverage ES6+ features like template literals for dynamic regular expressions. Always consider the performance implications of regular expressions, especially when dealing with large texts. Use online tools like regex101.com to test and debug your regular expressions.

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