为什么缓存的Regexp优于已编译的Regexp？

Question

这只是一个满足我好奇心的问题.但对我来说这很有意思.

我写了这个简单的小基准.它以随机顺序调用3个Regexp执行变体几千次:

基本上,我使用相同的模式,但以不同的方式.

1. 没有任何你的普通方式RegexOptions.从.NET 2.0开始,这些不会被缓存.但是应该"缓存",因为它保存在一个非常全局的范围内而不是重置.

2. 同 RegexOptions.Compiled

3. 调用静态Regex.Match(pattern, input),它在.NET 2.0中得到缓存

这是代码:

static List<string> Strings = new List<string>();        
static string pattern = ".*_([0-9]+)\\.([^\\.])$";

static Regex Rex = new Regex(pattern);
static Regex RexCompiled = new Regex(pattern, RegexOptions.Compiled);

static Random Rand = new Random(123);

static Stopwatch S1 = new Stopwatch();
static Stopwatch S2 = new Stopwatch();
static Stopwatch S3 = new Stopwatch();

static void Main()
{
  int k = 0;
  int c = 0;
  int c1 = 0;
  int c2 = 0;
  int c3 = 0;

  for (int i = 0; i < 50; i++)
  {
    Strings.Add("file_"  + Rand.Next().ToString() + ".ext");
  }
  int m = 10000;
  for (int j = 0; j < m; j++)
  {
    c = Rand.Next(1, 4);

    if (c == 1)
    {
      c1++;
      k = 0;
      S1.Start();
      foreach (var item in Strings)
      {
        var m1 = Rex.Match(item);
        if (m1.Success) { k++; };
      }
      S1.Stop();
    }
    else if (c == 2)
    {
      c2++;
      k = 0;
      S2.Start();
      foreach (var item in Strings)
      {
        var m2 = RexCompiled.Match(item);
        if (m2.Success) { k++; };
      }
      S2.Stop();
    }
    else if (c == 3)
    {
      c3++;
      k = 0;
      S3.Start();
      foreach (var item in Strings)
      {
        var m3 = Regex.Match(item, pattern);
        if (m3.Success) { k++; };
      }
      S3.Stop();
    }
  }

  Console.WriteLine("c: {0}", c1);
  Console.WriteLine("Total milliseconds: " + (S1.Elapsed.TotalMilliseconds).ToString());
  Console.WriteLine("Adjusted milliseconds: " + (S1.Elapsed.TotalMilliseconds).ToString());

  Console.WriteLine("c: {0}", c2);
  Console.WriteLine("Total milliseconds: " + (S2.Elapsed.TotalMilliseconds).ToString());
  Console.WriteLine("Adjusted milliseconds: " + (S2.Elapsed.TotalMilliseconds*((float)c2/(float)c1)).ToString());

  Console.WriteLine("c: {0}", c3);
  Console.WriteLine("Total milliseconds: " + (S3.Elapsed.TotalMilliseconds).ToString());
  Console.WriteLine("Adjusted milliseconds: " + (S3.Elapsed.TotalMilliseconds*((float)c3/(float)c1)).ToString());
}

每次我称之为结果都是这样的:

    Not compiled and not automatically cached:
    Total milliseconds: 6185,2704
    Adjusted milliseconds: 6185,2704

    Compiled and not automatically cached:
    Total milliseconds: 2562,2519
    Adjusted milliseconds: 2551,56949184038

    Not compiled and automatically cached:
    Total milliseconds: 2378,823
    Adjusted milliseconds: 2336,3187176891

所以你有它.不多,但差异大约7-8%.

这不是唯一的谜.我无法解释为什么第一种方式会慢很多,因为它永远不会被重新评估,而是保存在一个全局静态变量中.

顺便说一句,这是在.Net 3.5和Mono 2.2上,它们表现完全相同.在Windows上.

那么,任何想法,为什么编译的变体甚至会落后？

EDIT1:

修复代码后,结果现在如下所示:

    Not compiled and not automatically cached:
    Total milliseconds: 6456,5711
    Adjusted milliseconds: 6456,5711

    Compiled and not automatically cached:
    Total milliseconds: 2668,9028
    Adjusted milliseconds: 2657,77574842168

    Not compiled and automatically cached:
    Total milliseconds: 6637,5472
    Adjusted milliseconds: 6518,94897724836

这几乎也淘汰了所有其他问题.

谢谢你的回答.

Answer 1

在 Regex.Match 版本中，您正在寻找模式中的输入。尝试交换参数。

var m3 = Regex.Match(pattern, item); // Wrong
var m3 = Regex.Match(item, pattern); // Correct