>perl -wMstrict -le
"my $str = 'abcd';
 for my $rx (qr{(?=[^a-z])}, qr{(?![a-z])}, qr{\b}) {
   my @offsets;
   push @offsets, $-[1] while $str =~ m{ ($rx) }xmsg;
   if (@offsets) {
     print qq{$rx matches '$str' at offset(s) @offsets};
     }
   else {
     print qq{$rx does not match '$str'};
     }
   }
"
(?-xism:(?=[^a-z])) does not match 'abcd'
(?-xism:(?![a-z])) matches 'abcd' at offset(s) 4
(?-xism:\b) matches 'abcd' at offset(s) 0 4