pattern attribute is actually a HTML spec, not a Rails thing. You can use the
title attribute to give users a hint on to what format is expected.
<%= f.text_field :username, pattern: "([A-Za-z0-9\-\_]+)", title: "A username can only contain letters, numbers, hyphens and underscores" %>
Find more information here: https://html.spec.whatwg.org/multipage/input.html#the-pattern-attribute
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