"The question is more a technical one: I am re-readning (camel book), Regexp chapter. And I really want to know how to use this negative look behind."

"perlre - Perl regular expressions" has the basic information. Look for Look-Around Assertions under "Extended Patterns".

Possibly of more use if you are learning, try "perlretut - Perl regular expressions tutorial". There's a whole section on "Looking ahead and looking behind".

You can also use Super Search which will provide copious examples of usage.