From http://perldoc.perl.org/perlre.html#Regular-Expressions :

\b Match a word boundary

A word boundary (\b ) is a spot between two characters that has a \w on one side of it and a \W on the other side of it (in either order), counting the imaginary characters off the beginning and end of the string as matching a \W .

So, the "\b" will match "xxyy" if the following character is NOT [a-zA-Z0-9].

However, if the terminating character in the match is NOT a "word" character, as in your case - the terminating character is ")", then the "\b" expects the NEXT character to be a "word".

Hope this makes sense. If not, please experiment.