var files = await client.SearchAsync("/tax", "* Tax File *.csv", true);

using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
namespace FluentFTP
{
    public static class Extensions
    {
        public static async Task<IEnumerable<FtpListItem>> SearchAsync(this FtpClient client,
                                                                       string path, string filePattern, bool recursive = false)
        {
            filePattern = filePattern.Replace(@"\", @"\\")
                                     //escape regex special chars
                                     .RegexReplace(@"([+?|{[()^$#.])", @"\$1")
                                     .Replace("*", ".*?");
            filePattern = $"^{filePattern}$";           
            return await client.SearchFilesAsync(path, filePattern, recursive);
        }
        public static async Task<IEnumerable<FtpListItem>> SearchFilesAsync(this FtpClient client, 
                                                                            string path, string filePatternRegex, 
                                                                            bool recursive = false)
        {
            await client.SetWorkingDirectoryAsync(path);
            var listings = await client.GetListingAsync();
            var files = new List<FtpListItem>();
            if (recursive)
            {
                listings.ToList().ForEach(async item =>
                {
                    if (item.Type == FtpFileSystemObjectType.Directory)
                    {
                        files.AddRange(await client.SearchFilesAsync(item.FullName, filePatternRegex, recursive));
                    }
                });
            }
            files.AddRange(listings.Where(listing => listing.Type == FtpFileSystemObjectType.File &&
                                                        Regex.Match(listing.Name, filePatternRegex, RegexOptions.IgnoreCase | RegexOptions.Compiled).Success));
            return files;
        }
        public static string RegexReplace(this string str, string pattern, string replacement)
        {
            return Regex.Replace(str, pattern, replacement);
        }
    }
}