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Not all social media platforms and languages use the same hashtag rules as noted in the regular expression. The common hashtag format (letters, numbers, underscores) works best with Latin character set but doesn't work well with languages such as Chinese.
While an argument can be made in that screen readers should have advanced text processing capabilities such as hashtag announcements, I (personally) believe that it is really up to text to speech engines to define rules for these and give users a chance to edit speech dictionary entries. A screen reader is an information processing specialist in that it will consume whatever text information it can find. The job of actually announcing text in certain ways is really up to speech engines and humans to decide - screen readers can influence parts of that process, but it is really up to people to decide what to do with whatever they heard/read/saw.