gistfile1.rb

#!/usr/bin/env ruby
# encoding: utf-8

require 'nokogiri'

doc = Nokogiri::HTML('<p>abc</p>')
doc.inner_html = '<p>…</p>'
puts doc.inner_html

# output: <p>…</p>

########################################

#!/usr/bin/env ruby
# encoding: utf-8

require 'nokogiri'

original_string = '<div><p>abc</p></div>'
replacement_string = '<p>…</p>'

puts "original_string: #{original_string.inspect} (#{original_string.encoding.inspect})"
puts "replacement_string: #{replacement_string.inspect} (#{replacement_string.encoding.inspect})"
puts

doc = Nokogiri::HTML.fragment(original_string, 'UTF-8')
puts "original: #{doc.inner_html} (#{doc.inner_html.encoding.inspect})"

doc.css('div').first.inner_html = replacement_string
puts "replaced (1): #{doc.inner_html} (#{doc.inner_html.encoding.inspect})"

doc.inner_html = replacement_string
puts "replaced (2): #{doc.inner_html} (#{doc.inner_html.encoding.inspect})"

# output:
#
# original_string: "<div><p>abc</p></div>" (#<Encoding:UTF-8>)
# replacement_string: "<p>…</p>" (#<Encoding:UTF-8>)
# 
# original: <div><p>abc</p></div> (#<Encoding:UTF-8>)
# replaced (1): <div><p>…</p></div> (#<Encoding:UTF-8>)
# replaced (2): <p>…</p> (#<Encoding:UTF-8>)