IsHashtag

CSharp · Console

/Program.cs

using System;
using System.Linq;

					
public class Program
{
	public static void Main()
	{

		Predicate<string>f =
			s=>s[0]=='#'&s.Length>1&&(s[1]<48|s[1]>57)&s.Skip(1).All(x=>char.IsLetterOrDigit(x)|x=='_');
		
		
		Console.WriteLine(f("Hello World"));
		Console.WriteLine(f("#12thBday"));
		Console.WriteLine(f("#not-valid"));
		Console.WriteLine(f("#alsoNotValid!"));
		Console.WriteLine(f("#"));
		
		Console.WriteLine(f("#e"));
		Console.WriteLine(f("#_ABC"));
		Console.WriteLine(f("#thisisanunexpectedlylongstringxoxoxoxo"));
		Console.WriteLine(f("#USElection2016"));
	}
}