using System;
using System.Text.RegularExpressions;
public class Test
{
public static void Main()
{
string num = "5";
Console.WriteLine(Regex.Replace(num, @"\b\d\b", "0$&"));
num = "31";
Console.WriteLine(Regex.Replace(num, @"\b\d\b", "0$&"));
num = "a5b";
Console.WriteLine(Regex.Replace(num, @"(?<!\d)\d(?!\d)", "0$&"));
}
}
dXNpbmcgU3lzdGVtOwp1c2luZyBTeXN0ZW0uVGV4dC5SZWd1bGFyRXhwcmVzc2lvbnM7CgpwdWJsaWMgY2xhc3MgVGVzdAp7CglwdWJsaWMgc3RhdGljIHZvaWQgTWFpbigpCgl7CgkJc3RyaW5nIG51bSA9ICI1IjsKICAgICAgICBDb25zb2xlLldyaXRlTGluZShSZWdleC5SZXBsYWNlKG51bSwgQCJcYlxkXGIiLCAiMCQmIikpOwoKCQludW0gPSAiMzEiOwogICAgICAgIENvbnNvbGUuV3JpdGVMaW5lKFJlZ2V4LlJlcGxhY2UobnVtLCBAIlxiXGRcYiIsICIwJCYiKSk7CgoJCW51bSA9ICJhNWIiOwogICAgICAgIENvbnNvbGUuV3JpdGVMaW5lKFJlZ2V4LlJlcGxhY2UobnVtLCBAIig/PCFcZClcZCg/IVxkKSIsICIwJCYiKSk7CgoJfQp9