Regular Expression (RegExp) Quantifiers

In JavaScript, quantifiers define how many times a character, group, or character class must appear in a string for a match. They make regular expressions flexible and allow for precise or repeated pattern matching.

1. Basic Quantifiers

2. Greedy vs Non-Greedy (Lazy) Matching

By default, quantifiers are greedy, matching as much as possible. Adding ? after a quantifier makes it non-greedy (lazy), matching as little as possible.

const str = "<p>text</p><p>more</p>";
console.log(str.match(/<p>.*<\\/p>/));   // Greedy → ["<p>text</p><p>more</p>"]
console.log(str.match(/<p>.*?<\\/p>/g)); // Non-greedy → ["<p>text</p>", "<p>more</p>"]

3. Applying Quantifiers to Groups and Classes

Quantifiers can apply to:

• Single characters → /a+/ matches one or more 'a'
• Character classes → /[0-9]+/ matches one or more digits
• Groups → /(ab)+/ matches one or more "ab" sequences

console.log("ababab".match(/(ab)+/)); // ["ababab", "ab"]
console.log("12345".match(/[0-9]+/)); // ["12345"]

Practical Examples

Phone Number Matching:

const str = "123-456-7890";
console.log(str.match(/\\d{3}-\\d{3}-\\d{4}/)); // ["123-456-7890"]

Optional Protocol in URL:

const str = "<http://example.com>";
console.log(str.match(/https?:\\/\\//)); // ["http://"] (0 or 1 's')

Repeated Words:

const str = "hello hello hello";
console.log(str.match(/(hello\s*){2,}/)); // ["hello hello hello", "hello "]