using System;
using System.Text.RegularExpressions;

public class Test
{
    public static void Main()
    {
        string pattern = @"(?!^[ ]{0,}$)^\S{1,}$";
        string input = @"
   
abcd
ABCD1234
#$%^&*()_+={}
abc def
ABC 123
";
        RegexOptions options = RegexOptions.Multiline;
        
        foreach (Match m in Regex.Matches(input, pattern, options))
        {
            Console.WriteLine("'{0}' found at index {1}.", m.Value, m.Index);
        }
    }
}