The regular expression token ‹\b› is called a word boundary. It matches at the start or the end of a word. By itself, it results in a zero-length match. ‹\b› is an anchor, just like the tokens introduced in the previous section.

Strictly speaking, ‹\b› matches in these three positions:

To run a “whole words only” search using a regular expression, simply place the word between two word boundaries, as we did with ‹\bcat\b›. The first ‹\b› requires the ‹c› to occur at the very start of the string, or after a nonword character. The second ‹\b› requires the ‹t› to occur at the very end of the string, or before a nonword character.

Line break characters are nonword characters. ‹\b› will match after a line break if the line break is immediately followed by a word character. It will also match before a line break immediately preceded by a word character. So a word that occupies a whole line by itself will be found by a “whole words only” search. ‹\b› is unaffected by “multiline” mode or ‹(?m)›, which is one of the reasons why this book refers to “multiline” mode as “^ and $ match at line breaks” mode.

None of the flavors discussed in this book have separate tokens for matching only before or only after a word. Unless you wanted to create a regex that consists of nothing but a word boundary, these aren’t needed. The tokens before or after the ‹\b› in your regular expression will determine where ‹\b› can match. The ‹\b› in ‹\bx› and ‹!\b› could match only at the start of a word. The ‹\b› in ‹x\b› and ‹\b!› could match only at the end of a word. ‹x\bx› and ‹!\b!› can never match anywhere.

If you really want to match only the position before a word or only after a word, you can do so with lookahead and lookbehind. Recipe 2.16 explains lookahead and lookbehind. This method does not work with JavaScript and Ruby 1.8 because these flavors do not support lookbehind. The regex ‹(?<!\w)(?=\w)› matches the start of a word by checking that the character before the match position is not a word character, and that the character after the match position is a word character. ‹(?<=\w)(?!\w)› does the opposite: it matches the end of the word by checking that the preceding character is a word character, and that the following character is not a word character. It’s important to use negative lookaround with ‹\w› rather than positive lookaround with ‹\W› to check for the absence of a word character. ‹(?<!\w)› matches at the start of the string because there is no word character (or any character at all) before the start of the string. But ‹(?<=\W)› never matches at the start of the string. ‹(?!\w)› matches at the end of the string for the same reason. So our two lookaround constructs will correctly match the start of the string if the string begins with a word and the end of the string if it ends with a word.

‹\B› matches at every position in the subject text where ‹\b› does not match. ‹\B› matches at every position that is not at the start or end of a word.

Strictly speaking, ‹\B› matches in these five positions:

‹\Bcat\B› matches cat in staccato, but not in My cat is brown, category, or bobcat.

To do the opposite of a “whole words only” search (i.e., excluding My cat is brown and including staccato, category, and bobcat), you need to use alternation to combine ‹\Bcat› and ‹cat\B› into ‹\Bcat|cat\B›. ‹\Bcat› matches cat in staccato and bobcat. ‹cat\B› matches cat in category (and staccato if ‹\Bcat› hadn’t already taken care of that). Recipe 2.8 explains alternation.