以下是

Here's how the code from @Chris here stacks up against a regular expression test:

void Main()
{
    string input = "abcdefghijklmnopq";
    string first = "de";
    string last = "op";
    Regex re1 = new Regex("de(.*)op", RegexOptions.None);
    Regex re2 = new Regex("de(.*)op", RegexOptions.Compiled);

    // pass 1 is JIT preheat
    for (int pass = 1; pass <= 2; pass++)
    {
        int iterations = 1000000;
        if (pass == 1)
            iterations = 1;

        Stopwatch sw = Stopwatch.StartNew();
        for (int index = 0; index < iterations; index++)
            BetweenOfFixed(input, first, last);
        sw.Stop();
        if (pass == 2)
            Debug.WriteLine("IndexOf: " + 
                sw.ElapsedMilliseconds + "ms");

        sw = Stopwatch.StartNew();
        for (int index = 0; index < iterations; index++)
            BetweenOfRegexAdhoc(input, first, last);
        sw.Stop();
        if (pass == 2)
            Debug.WriteLine("Regex adhoc: " + 
                sw.ElapsedMilliseconds + "ms");

        sw = Stopwatch.StartNew();
        for (int index = 0; index < iterations; index++)
            BetweenOfRegexCached(input, first, last);
        sw.Stop();
        if (pass == 2)
            Debug.WriteLine("Regex uncompiled: " +
                sw.ElapsedMilliseconds + "ms");

        sw = Stopwatch.StartNew();
        for (int index = 0; index < iterations; index++)
            BetweenOfRegexCompiled(input, first, last);
        sw.Stop();
        if (pass == 2)
            Debug.WriteLine("Regex compiled: " +
                sw.ElapsedMilliseconds + "ms");
    }
}

public static string BetweenOfFixed(string ActualStr, string StrFirst,
    string StrLast)
{
    int startIndex = ActualStr.IndexOf(StrFirst) + StrFirst.Length;
    int endIndex = ActualStr.IndexOf(StrLast, startIndex);
    return ActualStr.Substring(startIndex, endIndex - startIndex);
}

public static string BetweenOfRegexAdhoc(string ActualStr, string StrFirst,
    string StrLast)
{
    // I'm assuming you don't replace the delimiters on every call
    Regex re = new Regex("de(.*)op", RegexOptions.None);
    return re.Match(ActualStr).Groups[1].Value;
}

private static Regex _BetweenOfRegexCached =
    new Regex("de(.*)op", RegexOptions.None);
public static string BetweenOfRegexCached(string ActualStr, string StrFirst,
    string StrLast)
{
    return _BetweenOfRegexCached.Match(ActualStr).Groups[1].Value;
}

private static Regex _BetweenOfRegexCompiled =
    new Regex("de(.*)op", RegexOptions.Compiled);
public static string BetweenOfRegexCompiled(string ActualStr, string StrFirst,
    string StrLast)
{
    return _BetweenOfRegexCompiled.Match(ActualStr).Groups[1].Value;
}

输出:

IndexOf: 1419ms
Regex adhoc: 7788ms
Regex uncompiled: 1074ms
Regex compiled: 682ms