Programming Perl, 4th Edition

Boundaries: The \b and \B Assertions

The \b assertion matches at any word boundary, defined as the position between a \w character and a \W character, in either order. If the order is \W\w, it’s a beginning-of-word boundary, and if the order is \w\W, it’s an end-of-word boundary. (The ends of the string count as \W characters here.) The \B assertion matches any position that is not a word boundary—that is, the middle of either \w\w or \W\W.

/\bis\b/   # matches "what it is" and "that is it"
/\Bis\B/   # matches "thistle" and "artist"
/\bis\B/   # matches "istanbul" and "so—isn't that butter?"
/\Bis\b/   # matches "confutatis" and "metropolis near you"

Because \W includes all punctuation characters (except the underscore), there are \b boundaries in the middle of strings like “isn't”, “[e-mail]”, “M.I.T.”, and “key/value”.

Inside a bracketed character class ([\b]), a \b represents a backspace rather than a word boundary.