$ perl -le '
> print
>    qq{->$_<-: },
>    m{(?x) ^ (?: \z | (?> \s* ) \D ) }
>    ? q{matches}
>    : q{no match}
>    for q{}, q{12ab}, q{cat}, q{  99balloons}, q{   dog};'
-><-: matches
->12ab<-: no match
->cat<-: matches
->  99balloons<-: no match
->   dog<-: matches
$