Switch with Regex

Hi,

I'm having some trouble replicating this powershell code in C# could anyone give me a hand or point me in the right direction?

Powershell:

    $FirstLetter = $Promoter.Substring(0,1)
    Switch -Regex ($FirstLetter)
    {
    "[A-G]" {"A - J"}
    "[K-Z]" {"K - Z"}
    }

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