Why does .NET's regex engine behave so bizarrely when I omit the "else" from a 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 that as the "else". In this case it would run as if the regex was (?(x)bx|(?=b))

What the **** is going on here? Is this intentional? It doesn't seem to be documented.

Edit: An issue has been created in the corefx repository: https://github.com/dotnet/corefx/issues/26787

Answer 1

I think it may be a mis-optimization. As Alternation Constructs in Regular Expressions points out:

Because the regular expression engine interprets expression as an anchor (a zero-width assertion), expression must either be a zero-width assertion (for more information, see Anchors) or a subexpression that is also contained in yes.

Your expression value satisfies neither of these constraints. I suspect some form of optimization where, since the expression isn't zero-width the input is advanced until the yes can potentially be satisfied (since that's the only pattern you've given the regex engine to work with)

As pointed out in the comments, since your expression isn't also contained in yes, the pattern can never match and so it's unlikely too much concern would be raised about the mis-optimization.