>perl -wMstrict -le
"my @strs = (qw(123x foo ***), '');
 ;;
 for my $s (@strs) {
   $s =~ m{ \d+ }xmsgp;
   printf qq{for '%s': pos == %s, \${^MATCH} eq '%s' \n}, $s,
     pos($s) // 'undefined', ${^MATCH} // 'undefined',
     ;
   }
"
for '123x': pos == 3, ${^MATCH} eq '123'
for 'foo': pos == undefined, ${^MATCH} eq 'undefined'
for '***': pos == undefined, ${^MATCH} eq 'undefined'
for '': pos == undefined, ${^MATCH} eq 'undefined'