Fórum regex only alphanumerics (except underscore), C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

// aljodav
// http://rextester.com/users/376

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            Match m1=Regex.Match("123ab456_",@"\W|_|^$");
            bool onlyAlphaNumerics1=!m1.Success; // False
            
            Match m2=Regex.Match("123ab456@",@"\W|_|^$");
            bool onlyAlphaNumerics2=!m2.Success; // False
            
            Match m3=Regex.Match("",@"\W|_|^$");
            bool onlyAlphaNumerics3=!m3.Success; // False
            
            Match m4=Regex.Match("123ab456AB",@"\W|_|^$");
            bool onlyAlphaNumerics4=!m4.Success; // True
            
            Console.WriteLine(onlyAlphaNumerics1);
            Console.WriteLine(onlyAlphaNumerics2);
            Console.WriteLine(onlyAlphaNumerics3);
            Console.WriteLine(onlyAlphaNumerics4);
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,14 sec, absolute running time: 0,16 sec, cpu time: 0,14 sec, average memory usage: 16 Mb, average nr of threads: 4