What AnonyMonk may be getting at is the sometimes tricky difference between \D and (?! \d):

• \D requires the presence of a non-digit character, so 'xyz1' will not match against /\d\D/;
• (?! \d) only requires the absence of a digit character, and the end of the string qualifies as this absence so 'xyz1' will match against /\d(?!\d)/ — as you have discovered!

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'xyz1';
 ;;
 print '/\d \D/ matches'      if $s =~ m{ \d \D      }xms;
 print '/\d (?! \d)/ matches' if $s =~ m{ \d (?! \d) }xms;
"
/\d (?! \d)/ matches
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