Hello misterperl,

I'm not sure how that works exactly...

From “Look-Around Assertions” in perlre#Extended-Patterns:

• (?!pattern)
  A zero-width negative look-ahead assertion. For example /foo(?!bar)/ matches any occurrence of "foo" that isn't followed by "bar".

If you have access to the Camel Book, see also Chapter 5: Pattern Matching, section “Fancy Patterns,” subsection “Lookaround Assertions.”

Hope that helps,