Regular Expressions in JavaScript with Examples: In Practice

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Regular Expressions in JavaScript with Examples: In Practice

Regular expressions (RegExp) are a powerful tool for searching, replacing, and validating strings. In JavaScript and Node.js, they are used everywhere: from form validation to log parsing. In this article, we will cover the basics of syntax, popular methods, and solve several practical tasks.

1. Creating a Regular Expression

In JavaScript, there are two ways to create a regular expression:

Literal notation (recommended): /pattern/flags
Constructor new RegExp('pattern', 'flags')

Example:

// Literal notationconst regexLiteral = /hello/i;

// Constructor (useful when the pattern is dynamic)const pattern = 'hello';const regexConstructor = new RegExp(pattern, 'i');

console.log(regexLiteral.test('Hello World')); // trueconsole.log(regexConstructor.test('HELLO'));    // true

2. Basic RegExp and String Methods

To work with regular expressions in JavaScript, two types of methods are used: methods of the RegExp object itself and string methods.

2.1 The `test()` Method

Checks if there is a match in the string. Returns true or false.

const emailRegex = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}$/;console.log(emailRegex.test('user@example.com')); // trueconsole.log(emailRegex.test('invalid-email'));    // false

2.2 The `exec()` Method

Searches for a match and returns an array with detailed information (or null). Useful for extracting data.

const str = 'Price: 1500 rubles, discount: 200 rubles';const priceRegex = /(\\d+)/g;let match;

while ((match = priceRegex.exec(str)) !== null) {    console.log(`Found number: ${match[1]}`);}// Output:// Found number: 1500// Found number: 200

2.3 The String `match()` Method

Works similarly to exec(), but is called on the string.

const text = 'JavaScript is cool! JS is used everywhere.';const result = text.match(/JS/g);console.log(result); // [ 'JS', 'JS' ]

2.4 The String `replace()` Method

Replaces found matches with the specified string or the result of a function.

const phone = '+7 (123) 456-78-90';const cleaned = phone.replace(/[\\s\\-\\(\\)\\+]/g, '');console.log(cleaned); // 71234567890

// Replacement using capture groupsconst date = '2024-03-15';const formatted = date.replace(/(\\d{4})-(\\d{2})-(\\d{2})/, '$3.$2.$1');console.log(formatted); // 15.03.2024

2.5 The String `search()` Method

Returns the index of the first match or -1.

const sentence = 'Learn JavaScript at PythonLib';const index = sentence.search(/javascript/i);console.log(index); // 6

3. Flags and Special Characters: Practical Examples

Flags change the search behavior. The most popular ones:

g — global search (finds all matches, not just the first).
i — case-insensitive search.
m — multiline mode (anchors ^ and $ work on each line).

Example with the m flag:

const multiLineText = `First linesecond lineThird line`;

// Without the m flagconsole.log(multiLineText.match(/^second/)); // null

// With the m flagconsole.log(multiLineText.match(/^second/m)); // [ 'second' ]