using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine(GetPrecisionFromFormatString("бред"));

    }

    private static int GetPrecisionFromFormatString(string customFormat)

    {

        if (string.IsNullOrWhiteSpace(customFormat))

        {

            return 0;

        }

        var regex = new Regex(@"^{\d+:(N|P|C)(?<precision>\d*)}$");

        var precisionString = regex.Match(customFormat).Groups["precision"].Value;

        int precision;

        return int.TryParse(precisionString, out precision) ? precision : 2;

    }

}