// show.html.erb
<%= form_tag('/dev/index', method: :post) do %>
<%= text_area_tag :content %>
<%= text_field_tag :kw_before %>
<%= text_field_tag :kw_after %>
<%= submit_tag("submit") %>
<% end %>

// dev_controller.rb
def index
  @content = params[:content]
  @content.gsub!(/#{params[:kw_before]}/, params[:kw_after])

  Userinfo.create(name: @content)
end

// index.html.erb
<%= @content %>