'booga'  =~ /[^.]*/            matches.
'booga.' =~ /[^.]*/            matches.

'booga'  =~ /(?=.*\.)/         does not match.
'booga.' =~ /(?=.*\.)/         matches.


'4.6.8'  =~ /[^.]*\.(\d)/      returns 6 in $1.
'4.6.8'  =~ /(?=.*\.)(\d)/     returns 4 in $1.
'4.6.8'  =~ /((?=.*\.))\1(\d)/ returns 8 in $2. .* matched a "4.6"
'4.6.8'  =~ /.*\.(\d)/         returns 8 in $1. Lookahead not needed.
'4.6.8'  =~ /(?:(?!\.).)*(\d)/ returns 6 in $1. You want neg lookahead