Word boundary assertion: \b, \B
A word boundary assertion checks if the current position in the string is a word boundary. A word boundary is where the next character is a word character and the previous character is not a word character, or vice versa.
Syntax
regex
\b
\B
Description
\b
asserts that the current position in the string is a word boundary. \B
negates the assertion: it asserts that the current position is not a word boundary. Both are assertions, so unlike other character escapes or character class escapes, \b
and \B
don't consume any characters.
A word character includes the following:
- Letters (A–Z, a–z), numbers (0–9), and underscore (_).
- If the regex is Unicode-aware and the
i
flag is set, other Unicode characters that get canonicalized to one of the characters above through case folding.
Word characters are also matched by the \w
character class escape.
Out-of-bounds input positions are considered non-word characters. For example, the following are successful matches:
js
/\ba/.exec("abc");
/c\b/.exec("abc");
/\B /.exec(" abc");
/ \B/.exec("abc ");
Examples
Detecting words
The following example detects if a string contains the word "thanks" or "thank you":
js
function hasThanks(str) {
return /\b(thanks|thank you)\b/i.test(str);
}
hasThanks("Thanks! You helped me a lot."); // true
hasThanks("Just want to say thank you for all your work."); // true
hasThanks("Thanksgiving is around the corner."); // false
Warning: Not all languages have clearly defined word boundaries. If you are working with languages like Chinese or Thai, where there are no whitespace separators, use a more advanced library like Intl.Segmenter
to search for words instead.
Specifications
Specification |
---|
ECMAScript Language Specification # prod-Assertion |
Browser compatibility
BCD tables only load in the browser