格式化数字字符串的最简单方法是什么？

Question

I have the following string: "12345678901", and I want it to be formatted as "123.456.789-01", I'm currently doing it like this:

string s = "12345678901";
string formatted = Int64.Parse(s).ToString("000.000.000-00"); // = "123.456.789-01"

Is there a better way of doing this, can I do this directly, without having to convert the string to int and back to string again?

Answer 1

Yes, you can reach your result without conversion to an integer and reformatting the integer to a string and it is even faster.

s = "12345678901";
StringBuilder sb = new StringBuilder(s);
int pos = s.Length - 2;
if (pos > 0) sb.Insert(pos, "-");
pos -= 3;
while (pos > 0)
{
    sb.Insert(pos, ".");
    pos -= 3;
}
string formatted = sb.ToString();

This is more or less what madrereflection has told you in comments with the only difference that the example here uses StringBuilder to avoid the continuous rebuild of the string. From simple benchmark it is three times faster than the current approach but of course more code is required.