? JavaScript Regular Expressions

⚡ Quick Overview

A Regular Expression (RegExp) is a pattern used to match character combinations in strings. In JavaScript, regex is widely used for validation, searching, and replacing text.

You can write regex using the literal form /pattern/flags or with the RegExp constructor. JavaScript then provides methods like test(), exec(), match(), and replace() to work with these patterns.

? Key Concepts

• Pattern – what you want to find (letters, digits, words, etc.).
• Flags – extra options like case-insensitive or global search.
• Character classes – shortcuts like \d, \w, \s.
• Anchors – match positions such as start (^) or end ($).
• Quantifiers – how many times a pattern repeats (+, *, {n,m}).
• Methods – regex and string functions that use patterns.

? Syntax and Basic Patterns

Regex syntax

/pattern/modifiers
// or using RegExp constructor:
new RegExp("pattern", "modifiers")

Basic patterns

/abc/        // Matches "abc"
\d           // Matches any digit (0–9)
\w           // Matches any word character (a-z, A-Z, 0–9, _)
\s           // Matches whitespace
.            // Any single character except newline

^abc         // Must start with "abc"
abc$         // Must end with "abc"
a|b          // Matches "a" or "b"
[abc]        // Matches "a", "b", or "c"
[^abc]       // Anything except "a", "b", or "c"
[a-z]        // Range: a to z

? Modifiers (Flags)

• i — Case-insensitive match.
• g — Global match (find all matches, not just first).
• m — Multi-line mode (affects ^ and $).

? Special Characters and Quantifiers

\d     // Digits
\D     // Non-digits
\w     // Word characters
\W     // Non-word characters
\s     // Whitespace
\S     // Non-whitespace
\b     // Word boundary
\B     // Non-word boundary
.      // Any character except newline

+      // One or more
*      // Zero or more
?      // Zero or one
{n}    // Exactly n
{n,}   // At least n
{n,m}  // Between n and m

? Code Examples with RegExp Methods

✅ test() – Boolean check

let pattern = /hello/i;
console.log(pattern.test("Hello World"));  // true

? exec() – Detailed match

let result = /(\d+)/.exec("Price: 100 dollars");
console.log(result[0]); // "100"

? match() – Get all matches

let str = "The rain in Spain";
let matches = str.match(/ain/g);
console.log(matches);  // ["ain", "ain"]

? replace() – Search and replace

let text = "Hello 123!";
let newText = text.replace(/\d+/, "***");
console.log(newText);  // "Hello ***!"

? Live Output and Explanation

? Common Use Cases

• Validating emails, phone numbers, and passwords.
• Extracting numbers or specific patterns from a string.
• Cleaning input (removing extra spaces, unwanted characters).
• Masking sensitive data (e.g. replacing digits in phone numbers).