Regex模式对几乎相同的输入字符串产生不同的结果

为什么在相同的模式下对相似的搜索字符串得到不同的行为?

请注意，下面是一个同事写的，不是我自己。

https://dotnetfiddle.net/veyasw

using System;
using System.Text.RegularExpressions;
public class Program
{
    static void MatchTest(string input, string pattern)
    {
        Console.WriteLine("pattern: " + pattern);
        Console.WriteLine("input: " + input + Environment.NewLine);
        Match match = Regex.Match(input, pattern);
        if (match.Success)
            Console.WriteLine("Match '{0}' at index {1}", match.Value, match.Index);
        else
            Console.WriteLine("Not match");
        Console.WriteLine("'r'n------'r'n");
    }
    static void DiffBehaviousTest() // (?(expression)yes) has different behavious. Sometime it matches with string empty.
    {
        /* if last character in word is digit
                match ab
        */
        string pattern = @"(?(.*'d'b)ab)";
        MatchTest("xy xya", pattern);
        MatchTest("xy xyz", pattern);
    }

    public static void Main()
    {
        DiffBehaviousTest();
    }
}

收益率:

pattern: (?(.*'d'b)ab)
input: xy xya
Match '' at index 5
------
pattern: (?(.*'d'b)ab)
input: xy xyz
Not match
------

背景阅读:这里有一个条件正则表达式(?(expression)yes|no)的例子-如果它匹配表达式，它寻找yes模式，否则它不寻找模式。然而，这里我们不提供no案例模式。

下面是一个不使用上面提到的|no条件的正则表达式(搜索:(?(Open)(?!))$)的示例。

    string pattern = @"(?(.*'d'b)agarbage)";
    MatchTest("xy xya", pattern);
    MatchTest("xy xyb", pattern);

    string pattern = @"(?(.*'d'b)bgarbage)";
    MatchTest("xy xya", pattern);
    MatchTest("xy xyb", pattern);

    string pattern = @"(?(.*'d)agarbage|bgarbage)";
    MatchTest("xy xya", pattern);
    MatchTest("xy xyb", pattern);

    string pattern = @"(?(.*'d)a|b)";
    MatchTest("xy xya", pattern);
    MatchTest("xy xyb", pattern);