http://wwj718.github.io/regex-demo.html --> CSharp Version

gistfile1.cs

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

namespace zh_clean_csharp
{
    class Program
    {
        static void Main(string[] args)
        {
                        string PATH = args[0];
                        string FILETYPE = args[1];


            string[] files = Directory.GetFiles(PATH);
            string[] filtered = FileFilter(files, FILETYPE);

            foreach (string file in filtered)
            {
                FilterAndOutput(file, FILETYPE);
                Console.WriteLine(GetFileName(file, FILETYPE) + "......OK");
            }

            Console.WriteLine("Press any key to continue...");
            Console.ReadKey(true);
            
        }


        public static void FilterAndOutput(string file, string FILETYPE)
        {
            string output_filename;

            output_filename = file.Replace(GetFileName(file, FILETYPE), GetFileName(file, FILETYPE) + "_Filtered");

            using (StreamWriter writer = new StreamWriter(output_filename))
            {
                using (StreamReader reader = new StreamReader(file))
                {
                    string line;
                    line = reader.ReadLine();

                    while (line != null)
                    {
                        var to_write = TextFilter(line);

                        writer.WriteLine(to_write);
                        line = reader.ReadLine();
                    }
                }
            }
        }



        public static string[] FileFilter(string[] files, string FILETYPE)
        {
            List<string> filtered_files = new List<string>();
            Regex pattern = new Regex(".+\\." +  FILETYPE.ToLower() + "$");

            foreach (string file in files)
            {
                if (pattern.IsMatch(file.ToLower()))
                    filtered_files.Add(file);
            }

            return filtered_files.ToArray();
        }

        private static string GetFileName(string file, string FILETYPE)
        {
            Regex pattern = new Regex("\\." + FILETYPE.ToLower() + "$");
            Match match;
            string filename;
            string cache;

            match = pattern.Match(file);

            cache = file.Remove(file.IndexOf(match.Value), match.Value.Length);
            filename = cache.Split('\\').Last<string>();

            return filename;
        }

        public static string TextFilter(string origin_data)
        {
            Regex pattern = new Regex("[\\u4e00-\\u9fa5].+$");
            string output_str;
            Match match;

            match = pattern.Match(origin_data);
            output_str = origin_data.Remove(origin_data.IndexOf(match.Value), match.Value.Length);

            return output_str;
        }
    }
}