Update hashtag regex

lib/auto_linker/parser.ex

@@ -45,7 +45,7 @@ defmodule AutoLinker.Parser do
   # https://www.w3.org/TR/html5/forms.html#valid-e-mail-address
   @match_email ~r/^[a-zA-Z0-9.!#$%&'*+\/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/u
 
-  @match_hashtag ~r/^(?<tag>\#\w+)/u
+  @match_hashtag ~r/^(?<tag>\#[A-Za-z][A-Za-z0-9_]+)/u
 
   @prefix_extra [
     "magnet:?",