Matches a word boundary. A word boundary matches the position where a word character is not followed or preceeded by another word-character. Note that a matched word boundary is not included in the match. In other words, the length of a matched word boundary is zero. (Not to be confused with [].)
Examples:
/m/ matches the 'm' in "moon" ;
/oo/ does not match the 'oo' in "moon", because 'oo' is followed by 'n' which is a word character;
/oon/ matches the 'oon' in "moon", because 'oon' is the end of the string, thus not followed by a word character;
/ww/ will never match anything, because a word character can never be followed by both a non-word and a word character.