From SEO perspective, which language code format in the URL is better and more correct?

Is the second format valid for the attribute hreflang in <link rel="alternate".....>? According to the Google guideline, the first format en-US is always described.

I have already done my website with the second format en_US. Is it worth to move to the first format if it is a better one.


Rather than trying to find an answer to choose underscores or dashes in URL, I would like to know about the correct language code usage in URL. For example, as I already used the en_US format, what is the correct usage?

<link rel="alternate" href="http://www.example.com/en_US" hreflang="en_US" /> <link rel="alternate" href="http://www.example.com/en_US" hreflang="en-US" />

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