I want to handle JSON::ParserError

Use resque to catch exceptions in Ruby (for example, JSON::ParserError.

require "json"

json='{"fate":"testarossa'
begin
  hash=JSON.parse(json)
  hash
rescue JSON::ParserError=>e
  $stderr.puts "ERROR:#{e}"
end

For resque, see http://docs.ruby-lang.org/ja/2.1.0/doc/spec=2fcontrol.html#begin.

I have already given you a basic answer, but I would like to give you an example of a possible additional solution.

require 'json'

s='{"abc":"aaa"'
s2 = '{"abc":"aaa"}'

t=JSON.parse(s)rescunil#t is substituted by nil
t=JSON.parse(s2)rescunil#t is substituted by {abc:'aaa'}

When I create a website and give strange characters, does it mean that I want to enter text in JSON format on the form and process the error?

In scaffold, for example, you created a model similar to the following:

 rails gscaffold JsonArticle body:text

In that case, you can add the following validation to app/models/json_article.rb to display an error when filling out the form.

class JsonArticle<ActiveRecord::Base
  values_each —body do | r, a, v |
    json = JSON.parse vrescunil
    r.errors.adda, "JSON format is incorrect" if json.nil?
  end
end