What are non-word boundary in regex (\B), compared to word-boundary?

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A word boundary (\b) is a zero width match that can match:

  • Between a word character (\w) and a non-word character (\W) or
  • Between a word character and the start or end of the string.

In Javascript the definition of \w is [A-Za-z0-9_] and \W is anything else.

The negated version of \b, written \B, is a zero width match where the above does not hold. Therefore it can match:

  • Between two word characters.
  • Between two non-word characters.
  • Between a non-word character and the start or end of the string.
  • The empty string.

For example if the string is "Hello, world!" then \b matches in the following places:

 H e l l o ,   w o r l d !
^         ^   ^         ^

And \B matches those places where \b doesn’t match:

 H e l l o ,   w o r l d !
  ^ ^ ^ ^   ^   ^ ^ ^ ^   ^

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