c:\@Work\Perl\monks>perl -wMstrict -le
"$_ = 'aaab';
 print qq{Matched 1 => '$_'} if /^[a]{2,}(?![b])+/;
 print qq{LastMatch1: '$&'};
"
(?![b])+ matches null string many times in regex; marked by <-- HERE
 in m/^[a]{2,}(?![b])+ <-- HERE / at -e line 1.
Matched 1 => 'aaab'
LastMatch1: 'aa'