Determines if the input string is indeed a valid four character double literal

i do have a question .i have to:determine if the input string is indeed a valid four character double literal 1. Consists of exactly the following characters: ’+’, ’-’, ’.’ (decimal point), and ’0’ through ’9’ 2. Either the ’+’ or ’-’ character may appear only as the first character 3. The ’.’ (decimal point) character must appear exactly once 4. All other characters must be the ’0’ through ’9’ characters

my big problems is to put the +- at the first character and . once