Underscore not found in regular expression character class for character code detection

[srividyut]

Underscore not found in regular expression character class for character code detection

/synapse/rest/media/v1/preview_url_resource.py

• When used in countries other than Europe and the United States, garbled characters are awkward on web pages that use certain character codes. As you know the names of these character codes, we also use underscores.
• The difference is 2 lines but only 2 character correction
• There may be a deep reason why there is no underscore