Signature regular expression

I ran into some random situations where emails were not parsed correctly. Part of the email was chopped out, for example:
`Hi Test,\n\n-This is a test`
Will result in `-This is a test` fragment being hidden and considered as part of signature which it should not be. After digging more into the problem, we figured out that:
`(?m)(--\s*$|__\s*$|\w-$)|(^(\w+\s*){1,3} ym morf tneS$)`
The first matching group is testing against `\w-`, which is causing the problem. I can't think out of a better way to solve the problem other than take `\w-$` part out so it opens to any suggestion.

從 issue 中顯示的簽名正規表示式開始，重現提供的電子郵件範例，以檢查該片段為何被分類為簽名。在決定如何處理 \w-$ 這個 alternative 後，確認比對行為，並驗證範例片段仍然可見。

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