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JavaScript RegExp ? Quantifier - GeeksforGeeks

JavaScript RegExp ? Quantifier

Last Updated : 11 Jul, 2025

The ? quantifier in JavaScript regular expressions specifies that the preceding element is optional. It matches zero or one occurrence of the element, making it a useful tool for flexible pattern matching.

JavaScript


 let regex = /colou?r/;
let str1 = "color";
let str2 = "colour";
let str3 = "colouur";
console.log(regex.test(str1));
console.log(regex.test(str2));
console.log(regex.test(str3));

The pattern /colou?r/ matches both "color" and "colour" because the u is optional. It does not match "colouur" because it has more than one u.

Syntax:

element?

element: The character, group, or character class to be made optional.

Key Points

Matches zero or one occurrence of the preceding element.
Useful for optional letters or segments in words.
Can apply to individual characters or grouped patterns.

Real-World Examples 1. Optional Characters in Words JavaScript


 let regex = /behaviou?r/;
console.log(regex.test("behavior"));  
console.log(regex.test("behaviour")); 
console.log(regex.test("behaviours"));

Matches "behavior" and "behaviour" but not "behaviours".

2. Optional Groups JavaScript


 let regex = /(?:https?:\/\/)?www\.\w+\.\w+/;
console.log(regex.test("https://www.example.com/")); 
console.log(regex.test("www.example.com"));         
console.log(regex.test("example.com"));

The https?:// group is optional, so the pattern matches URLs with or without "http://" or "https://".

3. Matching Optional Digits JavaScript


 let regex = /\d{3}-?\d{4}/;
console.log(regex.test("123-4567")); 
console.log(regex.test("1234567")); 
console.log(regex.test("123--4567"));

The - is optional, so the pattern matches phone numbers with or without a hyphen.

4. Making a Case-Insensitive Match JavaScript


 let regex = /[A-Z]?pple/i;
console.log(regex.test("Apple"));
console.log(regex.test("pple"));   
console.log(regex.test("aPple"));

The optional [A-Z] allows for matching "Apple" or "pple".

5. Validating Postal Codes JavaScript


 let regex = /\d{5}(-\d{4})?/;
console.log(regex.test("12345"));    
console.log(regex.test("12345-6789")); 
console.log(regex.test("123456789"));

Matches U.S. ZIP codes with or without the four-digit extension.

Common Patterns Using ?

Optional Characters in Words:

/colou?r/

Optional Group in a URL:

/(?:https?:\/\/)?www\.\w+\.\w+/

Phone Numbers with Optional Separators:

/\d{3}-?\d{4}/

Case Insensitivity for a Letter:

/[A-Z]?pple/i

Postal Codes with Optional Extensions:

/\d{5}(-\d{4})?/

Limitations

Matches Only Once: Unlike the * quantifier (zero or more) or + (one or more), ? matches at most one occurrence.
Limited for Repeating Patterns: Use other quantifiers (*, +, {}) for patterns that require multiple matches.

Why Use the ? Quantifier?

Flexibility: Handles optional elements gracefully.
Readability: Keeps patterns concise and easy to understand.
Real-World Use Cases: Essential for handling variable text structures like optional URL protocols, postal code extensions, or flexible word spellings.

Conclusion

The ? quantifier is a powerful and versatile tool for making elements optional in regular expressions, enabling flexible and precise pattern matching.

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