Update hashtag regex

If you stare into the abyss long enough you can find that Twitter's hashtag regex is exceedingly complex to catch edge cases like forgetting to put a space at the end of a sentence's punctuation: e.g., "hey guys this is cool.#awesome"

That's crazy. But you can find more details about it here: https://github.com/twitter/twitter-text/

The reality is that Twitter's hashtags have evolved over the years from a base ruleset to include some unicode chars and other things to be friendly to non-English speaking users. But it's really, really complicated. If we go back to Old School Twitter rules or "What English speaking users encounter", the rules are pretty simple:

• Alpha, Numeric, and underscores are allowed (no dashes or other punctuation!)
• The hashtag must start with an Alpha character

This regex change should match these expectations. This will ensure that #100 doesn't get recognized as a hashtag, but #go100 will.

This fixes #1 (closed)

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:?",