JavaScript Regular Expressions

Match patterns in strings.

Creating Regex

let pattern = /hello/;           // Literal notation
let pattern = new RegExp("hello"); // Constructor (for dynamic patterns)
let pattern = /hello/gi;         // With flags

Basic Methods

let pattern = /hello/i;

pattern.test("Hello World");      // true (boolean)
"Hello World".match(pattern);     // ["Hello"]
"Hello World".search(pattern);    // 0 (index)
"Hello World".replace(pattern, "Hi"); // "Hi World"

Character Classes

/\d+/.test("123");    // true (digits)
/\w+/.test("hello");  // true (word characters)
/\s+/.test(" ");      // true (whitespace)
/.llo/.test("hello"); // true (any character)

Uppercase variants match inverses: \D (non-digit), \W (non-word), \S (non-whitespace).

Quantifiers

/ab*/.test("a");      // true (zero or more b's)
/ab+/.test("a");      // false (one or more b's)
/ab?/.test("a");      // true (zero or one b)
/a{2,4}/.test("aaa"); // true (2 to 4 a's)

Anchors

/^hello/.test("hello world");  // true (start of string)
/hello$/.test("say hello");    // false (not at end)
/^hello$/.test("hello");       // true (exact match)

Remember: Use ^ and $ for validation to ensure the entire string matches.

Groups and Alternation

/(cat|dog)/.test("I have a cat"); // true (OR)

let match = "123-456-7890".match(/(\d{3})-(\d{3})-(\d{4})/);
console.log(match[1]); // "123" (capturing group)

Flags

Email Validation

let emailPattern = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
emailPattern.test("test@example.com"); // true
emailPattern.test("invalid");          // false

Phone Validation

let phonePattern = /^\+?[\d\s-]{10,}$/;
phonePattern.test("+1-555-123-4567"); // true
phonePattern.test("123");              // false

Password Validation

let passPattern = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$/;
passPattern.test("MyP@ss123");  // true
passPattern.test("weak");        // false

Uses lookaheads (?=...) to require lowercase, uppercase, digit, and min 8 chars.

Extracting Data

let text = "Date: 2024-01-15";
let match = text.match(/(\d{4})-(\d{2})-(\d{2})/);
console.log(match[1], match[2], match[3]); // "2024" "01" "15"

Best Practices

• Test thoroughly — Edge cases matter
• Use character classes — More readable
• Use anchors — For exact matching
• Keep patterns simple — Document complex ones

Common Mistakes

• Forgetting ^ and $ — Partial matches
• Not using g flag — Only finds first match
• Overcomplicating — Keep patterns simple
• Not escaping special chars — . * + ? ( ) [ ] { } \ ^ $ |