NullReferenceException when parsing regular expression

[abelbraaksma]

Description

The following code raises an NRE:

Regex("(?(?x)yesno)")
// System.NullReferenceException: Object reference not set to an instance of an object.

Expected: a normal ArgumentException as any invalid regex raises that. The error in this regex is that the conditional part missing an alternation character |. Correct would be: ?(?x)yes|no)

Note that (?(x)yesno) compiles fine, so there's an argument to be made for the above to compile fine too, as technically they should be equivalent (a conditional alternation with only a true-part).

(found while working on #38872, but not related to that issue per se)

[ghost]

Tagging subscribers to this area: @eerhardt, @pgovind
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[stephentoub]

This was fixed in .NET Core several versions ago. It now produces:
"System.Text.RegularExpressions.RegexParseException: Invalid pattern '(?(?x)yesno)' at offset 5. Unrecognized grouping construct."

[abelbraaksma]

My bad, I accidentally tested 4.8 only,which clearly didn't get that fix. Glad it's solved!

Btw, I don't think it should be an error, but I assume that's decided and a given now.