using System;
using System.Linq;
using System.Text.RegularExpressions;

public class Test {
    public static void Main() {
    // your code goes here

    var file_name = GetValidFileName("filename with space %1.txt");
    Console.WriteLine(GetValidFileName(file_name));

    }
    private static string GetValidFileName(string fileName) {
        // remove any invalid character from the filename.
        String ret = Regex.Replace(fileName.Trim(), "[^A-Za-z0-9_. ]+", "");
        return ret.Replace(" ", String.Empty);
    }
}