using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        // Define the column names

        string[] columns = {

            "column1", "column2", "column3", "column4", "column5",

            "column6", "column7", "column8", "column9", "column10"

        };

​

        // Create the pattern to match the columns with specific text or non-letters in between

        string pattern = string.Join("|", columns);

        string combinedPattern = @"\b(" + pattern + @")\b(?:(?:[^\p{L}]*)|(n/a|Not filled|uk|unknown))\b(" + pattern + @")\b";

​

        // Define the input string

        string input = "hello this is column2 n/a column3 is my test";

​

        // Replace the first column name with an empty string if the pattern matches

        string result = Regex.Replace(input, combinedPattern, m => m.Groups[3].Value);

​

        Console.WriteLine(result);

    }

}