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

public class Test
{
	public static void Main()
	{
		var texts = new List<string> { "Na#me","M2a_ny","Vari{sq}o@us","test [sq]uirrel h23ere!" };
    	var pattern = @"((?:(?!{sq})[A-Za-z0-9\s])+)|{sq}";
        foreach (var text in texts) {
        	var result = Regex.Matches(text, pattern).Cast<Match>()
        		.Aggregate("", (s, e) => s + e.Groups[1].Value, s => s);
        	Console.WriteLine(result);
        }
	}
}