Improving JavaScript processing of Ruby Regex

Question

I have a basic webapp that basically serves the same purpose as Rubular.com, but for gsub.

It runs some simple JavaScript, which POST's to a Sinatra app, which then updates page with the results.

index.haml

%h1 Gsubular
%p It's like Rubular! But for y'know...Gsub!

#gsub_div
  %h2 Gsub-ed String:
  #gsub_answer
  %form#gsub_form{:method=>"POST", :action=>"/"}
    %input#test_string{:name=>"test_string"}
    %span< .gsub!(
    %span< /
    %input#gsub_pattern_input{:name=>"pattern"}
    %span /,"
    %input#gsub_replacement_input{:name=>"replacement"}
    %span ")

The Sinatra POST handler:

# create
  post '/' do
    response = {}
    begin
      str = params[:test_string]
      regex_do_this = Regexp.new(params[:pattern])
      gsubbed_string = str.gsub!(regex_do_this, params[:replacement])
    rescue Exception => e
      response[:error] = e.message
      return response.to_json
    end
    response[:gsub_string] = []
    response[:gsub_string].push(gsubbed_string)
    response.to_json
  end

The JavaScript I reworked from someone else's JavaScript to just detect Regex looks like this:

$(function() {
  $("#gsub_pattern_input, #gsub_replacement_input, #test_string").bind('keyup', function(e){
    $.post('/', $(e.target).parents("form").serialize(), function(response){
      var results = eval("(" + response + ")");
      var result;
      $("#gsub_answer").empty();
      $("#gsub_error").empty();
      if(results.gsub_string){
        result = escapeHTMLEncode(results.gsub_string[0])
        if(result == "undefined")
          result = "no matches"
        if(result == "null")
          result = "[EMPTY STRING]"
        $("#gsub_answer").css('color', 'green');
        $("#gsub_answer").html(result);
      }
      else
        $("#gsub_answer").css('color', 'red');
        $("#gsub_answer").html(results.error);
    });
  })
})

function escapeHTMLEncode(str) {
 var div = document.createElement('div');
 var text = document.createTextNode(str);
 div.appendChild(text);
 return div.innerHTML;
}

// Hacky way of stopping enter from submitting...
$(document).ready(function() {
  $(window).keydown(function(event){
    if(event.keyCode == 13) {
      event.preventDefault();
      return false;
    }
  });
});

I'm not a Javascript expert, but it looks ok to me in general. I know that evals are evil, so that needs fixing.

Can anyone give me some guidance on how to clean this up a bit?

The full Github repo is here.