using System;
using System.Linq;

namespace RegExSpeedTest
{
    class Program
    {
        static void Main(string[] args)
        {
            string sText;
            using (System.IO.StreamReader streamReader = new System.IO.StreamReader(@"C:\Path\Files\longText.txt", System.Text.Encoding.GetEncoding(1251)))
            {
                sText = streamReader.ReadToEnd();
                streamReader.Close();
            };

            System.Text.RegularExpressions.Regex pReg = new System.Text.RegularExpressions.Regex(@"\D+", System.Text.RegularExpressions.RegexOptions.Compiled);
            DateTime t = DateTime.Now;
            string sDigits = pReg.Replace(sText, "");
            Console.WriteLine("Net time: {0} мсек; digits count: {1}; all count: {2}", (DateTime.Now - t).TotalMilliseconds, sDigits.Length, sText.Length);

            ////вариант2
            System.Text.StringBuilder sb = new System.Text.StringBuilder(sText);
            t = DateTime.Now;
            int y = 0;
            for (int x = 0; x < sb.Length; ++x)
            {
                  if (sb[x] >= '0' && sb[x] <= '9') sb[++y] = sb[x];
            }
            Console.WriteLine("Net time: {0} мсек; digits count: {1}; all count: {2}", (DateTime.Now - t).TotalMilliseconds, sb.ToString(0, y).Length, sText.Length);

            ////вариант3
            char[] sText2 = sText.ToArray();
            t = DateTime.Now;
            y = 0;
            for (int x = 0; x < sText.Length; ++x)
            {
                if (sText2[x] >= '0' && sText2[x] <= '9') sText2[++y] = sText2[x];
            }
            string sDigits2 = new string (sText2).Substring(0,y);
            Console.WriteLine("Net time: {0} мсек; digits count: {1}; all count: {2}", (DateTime.Now - t).TotalMilliseconds, sDigits2.Length, sText.Length);
            Console.ReadKey();
        }
    }
}

#include "pch.h"
#include <fstream>
#include <ctime>
#include <sstream>

int main()
{
   std::wifstream f(L"C:\\Path\\Files\\longText.txt");
   std::wstringstream ss;
   ss << f.rdbuf();
   std::wstring s = ss.str();
   wprintf(L"digits count: %zi\n", s.size());
   unsigned long long time0 = clock(); // начальное время
   int y = 0;
   for (int x = 0; x < s.size(); ++x)
   {
      if (s[x] >= L'0' && s[x] <= L'9')
      {
         s[y] = s[x];
         ++y;
      }
   }
   s.resize(y);
   unsigned long long time1 = clock();
   wprintf(L"time = %zi, digits count: %zi\n", time1 - time0, s.size());
   system("pause");
   return 0;
}

using System;
using System.Linq;

namespace RegExSpeedTest
{
    class Program
    {
        static char[] sText3;

        static void Main(string[] args)
        {
            string sText;
            using (System.IO.StreamReader streamReader = new System.IO.StreamReader(@"C:\Path\Files\longText.txt", System.Text.Encoding.GetEncoding(1251)))
            {
                sText = streamReader.ReadToEnd();
                streamReader.Close();
            };

            System.Text.RegularExpressions.Regex pReg = new System.Text.RegularExpressions.Regex(@"\D+", System.Text.RegularExpressions.RegexOptions.Compiled);
            DateTime t = DateTime.Now;
            string sDigits = pReg.Replace(sText, "");
            Console.WriteLine("V1-Net time: {0} мсек; digits count: {1}; all count: {2}", (DateTime.Now - t).TotalMilliseconds, sDigits.Length, sText.Length);

            ////вариант2
            System.Text.StringBuilder sb = new System.Text.StringBuilder(sText);
            t = DateTime.Now;
            int y = 0;
            for (int x = 0; x < sb.Length; ++x)
            {
                if (sb[x] >= '0' && sb[x] <= '9')
                {
                    sb[y] = sb[x];
                    ++y;
                }
            }
            Console.WriteLine("V2-Net time: {0} мсек; digits count: {1}; all count: {2}", (DateTime.Now - t).TotalMilliseconds, sb.ToString(0, y).Length, sText.Length);

            ////вариант3
            char[] sText2 = sText.ToArray();
            t = DateTime.Now;
            y = 0;
            for (int x = 0; x < sText.Length; ++x)
            {
                if (sText2[x] >= '0' && sText2[x] <= '9')
                {
                    sText2[y] = sText2[x];
                    ++y;
                }
            }
            string sDigits2 = new string (sText2).Substring(0,y);
            Console.WriteLine("V3-Net time: {0} мсек; digits count: {1}; all count: {2}", (DateTime.Now - t).TotalMilliseconds, sDigits2.Length, sText.Length);

            ////вариант4
            unsafe
            {
                sText3 = sText.ToArray();
                fixed (char* sText3 = Program.sText3)
                {
                    t = DateTime.Now;
                    y = 0;
                    for (int x = 0; x < sText.Length; ++x)
                    {
                        if (sText3[x] >= '0' && sText3[x] <= '9')
                        {
                            sText3[y] = sText3[x];
                            ++y;
                        }
                    }
                }
            }
            string sDigits3 = new string(sText3).Substring(0, y);
            Console.WriteLine("V4-Net time: {0} мсек; digits count: {1}; all count: {2}", (DateTime.Now - t).TotalMilliseconds, sDigits3.Length, sText.Length);
            Console.ReadKey();
        }
    }
}