In many cases other characters are also allowed, in particular accented characters used in many languages, but handling may vary from one client to another, and from time to time as standards evolve.
A discussion of hashtag standards suggests that if #Romeo&Juliet is used, different Twitter clients might link to #Romeo, #Romeo&, or #Romeo&Juliet.
It is considered acceptable to tag a post once when contributing to a specific conversation.
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Twitter uses a different syntax for Chinese characters and orthographies with similar spacing conventions: the hashtag contains unspaced characters, separated from preceding and following text by spaces (e.g.
'我 #爱 你' instead of '我#爱你') or by zero-width non-joiner characters before and after the hashtagged element, to retain a linguistically natural appearance (displaying as unspaced '我#爱你', but with invisible non-joiners delimiting the hashtag).
Hashtags are not case sensitive; a search for “#hashtag” will find “#Hash Tag”.
The use of embedded capitals (Camel Case) increases readability and avoids confusion; a (real) pen shop would be advised to use #Pen Island rather than all lower-case.