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
| Quantifier | Meaning | Example |
|---|---|---|
* |
Matches 0 or more occurrences of the preceding character/group/class | "caat".match(/ca*t/) → ["caat"] "ct".match(/ca*t/) → ["ct"] |
+ |
Matches 1 or more occurrences | "caat".match(/ca+t/) → ["caat"] "ct".match(/ca+t/) → null |
? |
Matches 0 or 1 occurrence (optional) | "cat".match(/ca?t/) → ["cat"] "ct".match(/ca?t/) → ["ct"] |
{n} |
Matches exactly n occurrences | "caaat".match(/ca{3}t/) → ["caaat"] "cat".match(/ca{3}t/) → null |
{n,} |
Matches n or more occurrences | "caaaat".match(/ca{2,}t/) → ["caaaat"] "cat".match(/ca{2,}t/) → null |
{n,m} |
Matches between n and m occurrences | "caaat".match(/ca{2,4}t/) → ["caaat"] "caaaaat".match(/ca{2,4}t/) → null |
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 "]