Why is underscore allowed in hashtags(e.g #hello_world) but not dash(e.g #hello-world)?

And why don't Twitter and other applications allow digit-only hashtags e.g #123? [Edit: got the answer to this in response]


The octothorpe/hash (#) symbol has been considered the symbol for "number" way before it was used for tagging in social media. "#1" has always been "number one". If Twitter allowed numeric-only hashtags, people would write things like "#123" to mean "number 123" but Twitter would mistakenly consider it as a hashtag.

  • 2
    Perhaps this is the same reason for no dashes? #123-456-1232, both numbers and dashes are commonly used in phone numbers – DasBeasto Aug 10 '16 at 13:01
  • How the fact that dashes are used in phone numbers is related to hyphen usage in hashtags? #foo-bar-baz is no phone number. – pfalcon Sep 6 '20 at 9:29

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