>perl -wMstrict -le
"my $rx = 'x\dy';
 for my $str (qw(xxy x23y x4y xy)) {
   print qq{match '$str'}    if $str =~ $rx;
   print qq{no match '$str'} if $str !~ $rx;
   }
"
no match 'xxy'
no match 'x23y'
match 'x4y'
no match 'xy'