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JavaScript RegExp \b Metacharacter - GeeksforGeeks

JavaScript RegExp \b Metacharacter

Last Updated : 11 Jul, 2025

The \b metacharacter in JavaScript regular expressions represents a word boundary, allowing you to match positions where a word begins or ends. A word boundary is the position between a word character (\w: letters, digits, or underscores) and a non-word character (\W: everything else, including spaces and punctuation).

JavaScript


 let str1 = "word";
let str2 = "wordplay";
let regex = /\bword\b/;
console.log(regex.test(str1)); 
console.log(regex.test(str2));

In str1, the entire word "word" is surrounded by boundaries.
In str2, "word" is part of "wordplay" and thus not bounded.

Syntax:

/\bpattern\b/

Key Points

Position-Based Matching: Matches at the start or end of a word, not the characters themselves.
Definition of Word Characters: Includes letters (A-Z, a-z), digits (0-9), and underscores (_).
Non-Consuming Match: Does not consume characters; only matches positions.

Real-World Examples 1. Matching Whole Words JavaScript


 let regex = /\bcats\b/;
let str1 = "cats are cute";
let str2 = "concatsenate";
console.log(regex.test(str1)); 
console.log(regex.test(str2));

Here, \b ensures "cats" is matched only as a whole word.

2. Finding Words at the Start or End JavaScript


 let regexStart = /\bstart/;
let regexEnd = /end\b/;

console.log(regexStart.test("start of the sentence")); 
console.log(regexEnd.test("the sentence ends here"));

Word boundaries allow matching at the start or end of words.

3. Splitting Words Using \b JavaScript


 let str = "one, two, three!";
let regex = /\b/;
console.log(str.split(regex));

Output

[ 'one', ', ', 'two', ', ', 'three', '!' ]

Splitting a string using \b results in parts separated at word boundaries.

4. Extracting Words JavaScript


 let regex = /\b\w+\b/g;
let str = "Extract words from this sentence.";
console.log(str.match(regex));

Output

[ 'Extract', 'words', 'from', 'this', 'sentence' ]

The \b metacharacter ensures only whole words are matched.

5. Case-Sensitive Matching JavaScript


 let regex = /\bhello\b/i;
console.log(regex.test("Hello world")); 
console.log(regex.test("hello-world")); 
console.log(regex.test("helloworld"));

Using \b ensures "hello" is recognized as a separate word, even with case-insensitive matching.

When Not to Use \b

Non-Word Characters: If your pattern includes symbols, \b may not work as expected.

JavaScript


 let regex = /\b#tag\b/;
console.log(regex.test("#tag"));

Here, # is a non-word character, so \b doesn't match.
Languages With Special Word Characters: For non-Latin scripts or custom word delimiters, \b may need additional handling.

Why Use the \b Metacharacter?

Precise Matching: Ideal for matching whole words in texts, avoiding partial matches.
Flexible Positioning: Works at the start, end, or middle of a string to identify word boundaries.
Text Processing: Useful for parsing or analyzing structured or unstructured text data.

Conclusion

The \b metacharacter is a powerful tool for identifying and isolating words in a string, ensuring precision and efficiency in regular expression operations.

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