JS RegExp

JavaScript RegExp: Pattern Matching, Validation, & Matching Methods

Regular expressions (RegExp) are patterns used to match character combinations in strings. In JavaScript, regular expressions are objects created via literal notation (/pattern/flags) or the RegExp constructor.

RegExp Building Blocks Architecture

flowchart TD
    RegExpSyntax["/pattern/flags"] --> Pattern["Pattern: ^[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,}$"]
    RegExpSyntax --> Flags["Flags: g (global), i (ignoreCase), m (multiline), u (unicode)"]

Key RegExp Methods Comparison

Practical Code Example

// Demonstrating RegExp validation, extraction, and replacement

// 1. Email Validation with regex.test()
const emailPattern = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;
const sampleEmail = "[email protected]";
console.log(`Is valid email? ${emailPattern.test(sampleEmail)}`); // true

// 2. Extracting Data using Capture Groups and exec()
const datePattern = /(\d{4})-(\d{2})-(\d{2})/; // YYYY-MM-DD
const textLog = "Created on 2026-08-13 in system log.";
const matchResult = datePattern.exec(textLog);

if (matchResult) {
  console.log(`Full Match: ${matchResult[0]}`); // "2026-08-13"
  console.log(`Year: ${matchResult[1]}, Month: ${matchResult[2]}, Day: ${matchResult[3]}`);
}

// 3. String Replacement with RegExp
const formattedPhone = "123-456-7890".replace(/-/g, ""); // Remove all hyphens
console.log(`Clean Phone Digits: ${formattedPhone}`); // "1234567890"

Best Practices & Gotchas

• Use RegExp Literals for Static Patterns: Prefer literal notation /pattern/g when the regular expression pattern is known at authoring time.
• Use RegExp Constructor for Dynamic Patterns: Use new RegExp(dynamicString, "i") when building regexes dynamically from user input variables. Remember to double-escape backslashes ("\d+").
• Reset lastIndex on Global (/g) Regexes: Regular expressions with the /g flag maintain stateful lastIndex properties when calling .test() repeatedly.

Self-Check Challenge

Write a regular expression /^\d{5}$/ to validate 5-digit postal codes.