Regular Expression Basic Syntax Metacharacters

\d matches numbers 0-9 

code example

string s = Console.ReadLine();
string pattern = @"^\d*$";
//\d means a number from 0-9 ^ means start with a number $ means end with a number. * means 0~n characters.
bool isMatch = Regex.IsMatch(s, pattern);

Console.WriteLine(isMatch);
Console.ReadKey();