What is Regex?

Regular expressions (regex) are patterns used to match sequences of characters in strings. They are powerful tools for searching, validating, and manipulating text.

Creating a Regex:

1. Literal Syntax:

Regex patterns are written between slashes (/pattern/).

Syntax: (/pattern/).

2. Constructor Syntax:

Use the RegExp constructor to create a regex.

Syntax: let regex = new RegExp(“hello”);

Special Characters in Regex:

1. Anchors:

• ^: Matches the beginning of a string.
• $: Matches the end of a string.

let regex = /^hello/; // Matches "hello" only if it’s at the start.
console.log(regex.test("hello world")); // true
console.log(regex.test("world hello")); // false

2. Character Classes:

• .: Matches any single character except a newline.
• \d: Matches any digit (0-9).
• \w: Matches any word character (alphanumeric + underscore).
• \s: Matches any whitespace character (space, tab).
• \D, \W, \S: Match the opposite of \d, \w, and \s.

let regex = /\d/; // Matches any digit.
console.log(regex.test("abc123")); // true

3. Quantifiers:

• *: Matches 0 or more occurrences.
• +: Matches 1 or more occurrences.
• ?: Matches 0 or 1 occurrence.
• {n}: Matches exactly n occurrences.
• {n,}: Matches at least n occurrences.
• {n,m}: Matches between n and m occurrences.

let regex = /a{2,4}/; // Matches "aa", "aaa", or "aaaa".
console.log(regex.test("aaa")); // true

4. Groups and Ranges:

• (abc): Matches the exact group abc.
• [a-z]: Matches any character in the range a to z.
• [^a-z]: Matches any character NOT in the range a to z.

let regex = /[A-Z]/; // Matches any uppercase letter.
console.log(regex.test("Hello")); // true

5. Escaping Special Characters:

• Use \ to escape special characters like ., *, ?, etc.

let regex = /\./; // Matches a literal dot.
console.log(regex.test("1.23")); // true