Why does the regex.NET mechanism behave so strangely when I omit the "else" from the conditional group?

Question

code:

Match match = Regex.Match("abc", "(?(x)bx)");
Console.WriteLine("Success: {0}", match.Success);
Console.WriteLine("Value: \"{0}\"", match.Value);
Console.WriteLine("Index: {0}", match.Index);

Output:

Success: True
Value: ""
Index: 1

It seems that a conditional group without an "else" expression will instead create a lookahead from the first character of the "if" expression and use it as an "else". In this case, it will work as if the regular expression were(?(x)bx|(?=b))

What's going on here? Is it intentional? This does not seem to be documented.

Edit: The problem was created in the corefx repository: https://github.com/dotnet/corefx/issues/26787

Answer 1

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