JavaScript RegExp \B Metacharacter

The \B metacharacter in JavaScript regular expressions matches a position where a word boundary does not exist. It is essentially the opposite of the \b (word boundary) metacharacter.

let regex = /\Bcat\B/;
let str1 = "concat";
let str2 = "cat";
console.log(regex.test(str1));
console.log(regex.test(str2)); 

false
false

The pattern \Bcat\B matches "cat" only if it is not at the beginning or end of a word.

Syntax:

let regex = /\Bpattern\B/;

• \B: Matches a position where no word boundary (\b) exists.

Key Points

• Matches positions inside a word, not at the start or end.
• Useful for precise control over word-boundary-sensitive patterns.
• Often used to restrict matches to non-boundary contexts.

Real-World Examples

1. Matching Inside Words

let regex = /\Bcat\B/;
let str1 = "concatenate";
let str2 = "catapult";
console.log(regex.test(str1)); 
console.log(regex.test(str2)); 

true
false

The \Bcat\B matches "cat" only within another word, such as "concatenate," but not when it starts or ends a word.

2. Restricting Prefix or Suffix Matches

let regex = /\Bing/;
let str1 = "running";
let str2 = "ing";
console.log(regex.test(str1)); 
console.log(regex.test(str2)); 

true
false

The \Bing matches "ing" only when it is not at the start of the word.

3. Highlighting Non-Boundary Matches

let regex = /\Bis\B/g;
let str = "This island is beautiful.";
let matches = str.match(regex);
console.log(matches);

null

The \B ensures only the "is" within "This" is matched, not the standalone "is."

4. Removing Characters Within Words

let str = "hello1world2";
let regex = /\B\d\B/g;
let result = str.replace(regex, "");
console.log(result);

helloworld2

The \B\d\B matches digits (\d) that are surrounded by word characters and removes them.

5. Preventing Word Boundary Matches

let regex = /\Bis/;
let str = "island";
console.log(regex.test(str));

false

The \Bis matches "is" only when it is not at the start of a word.

Common Patterns Using \B

• Match Inside Words Only:

/\Bword\B/

• Find Suffixes Without Word Endings:

/\Bing/

• Remove Inner Digits:

str.replace(/\B\d\B/g, "");

• Highlight Internal Patterns:

/in\B/g

• Prevent Full Word Matches:

/prefix\B/

Limitations

• Context-Specific: Only useful in cases where non-boundary detection is necessary.
• Non-Intuitive: Can be confusing when combined with quantifiers or other regex elements.

Why Use \B Metacharacter?

• Precise Matching: Helps identify patterns inside words, avoiding boundaries.
• Text Parsing: Useful for extracting substrings or cleaning text data.
• Control Over Context: Provides fine-grained control in scenarios where boundary matching (\b) is insufficient.

Conclusion

The \B metacharacter is a powerful but specialized tool in regex, enabling precise pattern matching where word boundaries do not exist.

Recommended Links:

• JavaScript RegExp Complete Reference
• JavaScript Cheat Sheet-A Basic guide to JavaScript
• JavaScript Tutorial