using System;
using System.Text;

public static class Identifier
{
    public static string Clean(string identifier)
    {
        StringBuilder sbIdentifier = new StringBuilder();

        for (int i = 0;i < identifier.Length; i++)
        {
            if (identifier[i] == ' ')
            {
               sbIdentifier.Insert(i, '_');
               Console.WriteLine("SPACE");
            }
            else if (Char.IsControl(identifier, i))
            {
                sbIdentifier.Insert(i, "CTRL");
                Console.WriteLine("IsControl");
            }
            else
            {
                sbIdentifier.Insert(i, identifier[i]);
                Console.WriteLine("NOT NULL");
            }
        }

         return sbIdentifier.ToString();

    }
}