How i write the regular Expression which only accepts alphabates +one underscrore only from speical charaters

Hi all ,
 
How i write the regular Expression which only accepts the alpha numeric plus one hypen and underscore characters  only. not others
like City1_street
 
i tries the below but it is not working what iam doing wrong
"^[a-zA-Z]+$"
 
Best Regrds 
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