[API Proposal]: add regex for pattern matching

[balakhaniyan]

Background and motivation

Hello
Can dotnet team add a feature to pattern matching to use RegEx? (with is keyword)

API Proposal

I think there is no need to submit an API proposal

API Usage

if (data is @"\s+") {

}

Alternative Designs

No response

Risks

No response

[huoyaoyuan]

This is a C# language feature and belongs to https://github.com/dotnet/csharplang/discussions.

Testing IsMatch itself will definitely not be a language feature. It's just a shorthand of one method Regex.IsMatch. This falls under user-defined active pattern, which is discussed at #277 and #4131 .
The more interesting case is to extract the matched captures directly as variables, like the opposite of string interpolation.

[Unknown6656]

The problem with your proposal is, that your example is already valid code:
https://sharplab.io/#v2:EYLgtghglgdgNAFxAJwK4wD4AEBMBGAWAChjis8AGAAgGcqBeKrLAIgB0aBqFgblKPIBOABR0odZuy4sAlHwF4RYiaw4BvPAF9ZPIA==

Meaning we cannot change how the existing expression of the type ... is "string" behaves, as it would break existing code.

[balakhaniyan]

OK, but what about:

if (str is case @"\s+")

or

if (str case @"\s+")

?

[Unknown6656]

That would indeed be a possibility, however, I am not sure whether the introduction of a new syntax is worth the effort. Additionally, how would you propose to implement the usage of RegexOptions in that case?

I, for example, nearly always use a combination of the following RegexOptions flags: IgnoreCase, CultureInvariant, Compiled, and NonBacktracking.

Should a syntax à la JavaScript be introduced?

if (str is /\s+/i)
    ...

[jnm2]

If we got user-defined positional patterns (active patterns), we could help ourselves to regex patterns, ignore-case patterns, and anything we could dream of.

Currently championed at #4131.

[balakhaniyan]

And also some of them can be used in Expression Trees :), I know it has some challenges, but it is highly recommended

[jnm2]

Here's what expression trees are up against: #4727 (comment)