using System;
using System.Collections.Generic;
using System.Globalization;
using System.Text;
using System.Text.RegularExpressions;
using System.Reflection;
public class MainClass{
public static void Main(){
Regex r = new Regex("^\\d+");
string[] ss = new string[] { "123abc", "abc123", "123", "abc", "a123bc" };
foreach (string s in ss)
Console.WriteLine(r.IsMatch(s));
}
}
True
False
True
False