How to find multiple substring matches within a string, alter substring enclosures

Question

I am trying to parse a string of HTML with ruby, this string contains multiple <pre></pre> tags, I need to find and encode all < and > brackets in between each of these elements.

Example: 

string_1_pre = "<pre><h1>Welcome</h1></pre>"

string_2_pre = "<pre><h1>Welcome</h1></pre><pre><h1>Goodbye</h1></pre>"

def clean_pre_code(html_string)
 matched = html_string.match(/(?<=<pre>).*(?=<\/pre>)/)
 cleaned = matched.to_s.gsub(/[<]/, "&lt;").gsub(/[>]/, "&gt;")
 html_string.gsub(/(?<=<pre>).*(?=<\/pre>)/, cleaned)
end

clean_pre_code(string_1_pre) #=> "<pre>&lt;h1&gt;Welcome&lt;/h1&gt;</pre>"
clean_pre_code(string_2_pre) #=> "<pre>&lt;h1&gt;Welcome&lt;/h1&gt;&lt;/pre&gt;&lt;pre&gt;&lt;h1&gt;Goodbye&lt;/h1&gt;</pre>"

This works as long as html_string contains only one <pre></pre> element, but not if there are multiple.

I would be open to a solution that utilizes Nokogiri or similar, but couldn't figure how to make it do what I want.

Please let me know if you need any additional context.

Update: This is possible only with Nokogiri, see accepted answer.

Answer 1

@zstrad44 Yes you can get it done by using Nokogiri. Here is my version of code which I develop from your version and this will give you the result you want for multi pre tags in the string.

def clean_pre_code(html_string)
  doc = Nokogiri::HTML(html_string)
  all_pre = doc.xpath('//pre')
  res = ""
  all_pre.each do |pre|
    pre = pre.to_html
    matched = pre.match(/(?<=<pre>).*(?=<\/pre>)/)
    cleaned = matched.to_s.gsub(/[<]/, "&lt;").gsub(/[>]/, "&gt;")
    res += pre.gsub(/(?<=<pre>).*(?=<\/pre>)/, cleaned)
  end
  res
end

I would recommend you yo read Nokogiri Cheatsheet to have a better understanding of the methods I used in the code. Happy coding! Hope I could help