Some regex errors are misleading and could be improved

[abelbraaksma]

Description

Going over the possible exceptions for regexes (#38872) I noticed that a few appear to be raised that could be improved upon.

Better to raise MalformedNameRef

Regex(@"\k<>")
// raises:
// Framework: parsing "\k<>" - Unrecognized escape sequence \k.
// Core: Invalid pattern '\k<>' at offset 2. Unrecognized escape sequence \\k.

Regex(@"(?<xfo)")
// raises:
// Framework: parsing "(?<xfo)" - Invalid group name: Group names must begin with a word character.
// Core: Invalid pattern '(?<xfo)' at offset 6. Invalid group name: Group names must begin with a word character.

Both the above would be clearer if they raised MalformedNameRef. \k<...> is a valid way to reference a group, so MalformedNameRef just seems more logical.

Also, the 2nd case above, the text says "must begin with a word character", which is precisely what already happens in the regex...

Also, the Core version says \\k is not a recognized escape sequence, but there's no \\k in the expression (which would be valid), only \k.

Text is too specific or wrong

Not an {x, y} quantifier here:

Regex(@"((*foo)bar)")
Regex(@"?(x)?true|false)")
// raises:
// Framework: parsing "((*foo)bar)" - Quantifier {x,y} following nothing.
// Core: Invalid pattern '((*foo)bar)' at offset 3. Quantifier {x,y} following nothing.

Not a capture group in this expression:

Regex(@"x{234567899988}")
// raises:
// Framework: parsing "x{234567899988}" - Capture group numbers must be less than or equal to Int32.MaxValue.
// Core: Invalid pattern 'x{234567899988}' at offset 12. Capture group numbers must be less than or equal to Int32.MaxValue.

Confusing error, but not wrong

Regex(@"[--[b] ")
// raises:
// Framework: parsing "[--[b] " - A subtraction must be the last element in a character class.
// Core: Invalid pattern '[--[b] ' at offset 6. A subtraction must be the last element in a character class.

But I would find UnterminatedBracket clearer here, as non-termination of brackets or parens should be high-prio errors before the more special errors.

Unclear text when alternation is malformed

Regex(@"(x)(?(3x|y)")
// raises
// Framework: parsing "(x)(?(3x|y)" - (?(3) ) malformed.
// Core: Invalid pattern '(x)(?(3x|y)' at offset 8. (?(3) ) malformed.

I think this can be improved by making this error "conditional alternation malformed". Also, printing "(?(3) )" doesn't help here, as it is no present like that in the original expression.

But given the expression, I'd rather expect it to raise TooFewParentheses.

More info

Considering that Regex has been given so much attention lately (thanks so much for the perf improvements!!!), I figured why not also give the errors some scrutiny ;).

Edited: added full exception messages for both .NET Core and .NET Framework following up on #39075 (comment). The specific part is, as can be seen, the same for either environment.

Tested with:

• .NET Framework 4.8 (specifically: ".NET Framework 4.8.4069.0")
• .NET Core 3.1 (specifically: ".NET Core 3.1.3")

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[stephentoub]

Thanks for the report, but what version are you running / testing this on?

With the current code, for example,
@"(x)(?(3x|y)"
produces
Invalid pattern '(x)(?(3x|y)' at offset 8. (?(3) ) malformed.

[abelbraaksma]

Oh, I ran them under newest VS inside FSI window. I think that uses 4.8. I should've tested these with Core as well, I guess.

But, @stephentoub, you're showing the same error I did ;), except for the window dressing, which apparently got updated.

[stephentoub]

I should've tested these with Core as well, I guess

Yes :-)

you're showing the same error I did

The core of the text is the same for these examples. But there's additional description around it, such as the position, to help clarify.

[abelbraaksma]

@stephentoub, to prevent further confusion, I've updated the text above with the errors from both Framework and Core.

[stephentoub]

Cool, thanks :-)