for ( reverse 1..20 ) {
    $re .= sprintf "\\d{%d} {%d}|", $_, 20-$_;
}
chop $re;
$re = qr/^(?:$re)$/;
print $re, $/;

@tests = (
    '01234567890123456789', # OK
    '123                 ', # OK
    '123               ',   # NOK
    '123 123             ', # NOK
    '                    ', # NOK
    '123c                ', # NOK
);

for(@tests){
    print m/$re/ ? "'$_' #OK\n" : "'$_' #NOK\n"
}

__DATA__
(?-xism:^(?:\d{20} {0}|\d{19} {1}|\d{18} {2}|\d{17} {3}|\d{16} {4}|\d{15} {5}|\d{14} {6}|\d{13} {7}|\d{12} {8}|\d{11} {9}|\d{10} {10}|\d{9} {11}|\d{8} {12}|\d{7} {13}|\d{6} {14}|\d{5} {15}|\d{4} {16}|\d{3} {17}|\d{2} {18}|\d{1} {19})$)
'01234567890123456789' #OK
'123                 ' #OK
'123               ' #NOK
'123 123             ' #NOK
'                    ' #NOK
'123c                ' #NOK