$max_lhs = $precision - $scale;
$max_rhs = $scale;

$float =~ m/
            \A             # beggining of string
            \d{0,$max_lhs} # 0 to $max_lhs digits
            \.             # decimal point
            \d{0,$max_rhs} # 0 to $max_rhs digits
            \z             # end of string
           /xms;