sub untaint
{
my ($arg) = @_;
$$arg =~ m|^(.*)$|;

    if ($$arg eq "")
    { $$arg = $1; return 1; }

    if (($1 =~ m|^\.\.\/|) or ($1 =~ m|\/\.\.\/|))
    { return undef; }

    else { $$arg = $1; }
}