c:\@Work\Perl>perl -wMstrict -le
"use 5.010;
 ;;
 my $s = 'foo 1 foo  2 foo   3 4 x 567 foo foo 8 foo9';
 my @matches = $s =~ m{ (?> foo \s+ \d+ (*SKIP)(*FAIL))? \s+ \d+}xmsg;
 printf qq{'$_'  } for @matches;
"
' 4'  ' 567'