Allowing only one tag per post

I found this code in another post:   # clean string to remove spaces and force lowercase   def self.clean_string(string)     (string.downcase).gsub(" ","")   end My form:   <% form_for @post, :html => { :multipart => true } do |f| %>   #...     <%= f.label 'Topic' %><br />     <%= f.text_field :tag_list %>   #...   <% end %> I could use something like this in a callback to remove commas from the input on the text_field correct?