sub is_valid_string
{
    my $string = shift;

    # Handle the empty case
    return 0 unless length $string;

    # Preserve trailing empty fields
    my @string = split ',', $string, -1;

    # No commas at the beginning, end, or next to one another.
    return 0 if grep { !$_ } @string;

    return 0 if grep { /[^a-zA-Z]/ } @string;

    return 1;
}