Prevent Ruby from changing & to &? - ruby-on-rails

Question

I need to display some superscript and subscript characters in my webpage title. I have a helper method that recognizes the pattern for a subscript or superscript, and converts it to &sub2; or ²
However, when it shows up in the rendered page's file, it shows up in the source code as:
&sub2;
Which is not right. I have it set up to be:
<% provide(:title, raw(format_title(#hash[:page_title]))) %>
But the raw is not working. Any help is appreciated.
Method:
def format_title(name)
label = name
if label.match /(_[\d]+_)+|(~[\d]+~)+/
label = label.gsub(/(_([\d]+)_)+/, '&sub\2;')
label = label.gsub(/(~([\d]+)~)+/, '&sup\2;')
label.html_safe
else
name
end
end
I have even tried:
str.gsub(/&/, '&')
but it gives back:
&amp;sub2;

Answer 1

You can also achieve this with Rails I18n.
<%= t(:page_title_html, scope: [:title]) %>
And in your respective locale file. (title.en.yml most probably):
title:
page_title: "Title with ²"
Here is a chart for HTML symbols regarding subscript and superscripts.
For more information check Preventing HTML character entities in locale files from getting munged by Rails3 xss protection
Update:
In case you need to load the page titles dynamically, first, you'll have to install a gem like Page Title Helper.
You can follow the guide in the gem documentation.

Answer 2

There are two of issues with your example, one is of matter and the other is just a coincidence.
The first issue is you are trying to use character entities that do not actually exist. Specifically, there are only ¹, ² and ³ which provide 1, 2 and 3 superscript symbols respectively. There is no such character entity as &sup4; nor any other superscript digits. There are though bare codepoints for other digits which you can use but this would require a more involved code.
More importantly, there are no subscript character entities at all in HTML5 character entities list. All subscript digits are bare codepoints. Therefore it makes no sense to replace anything with &sub2; or any other "subscript" digit.
The reason you didn't see your example working is due to the test string you chose. Supplying anything with underscores, like _2_mystring will be properly replaced with &sub2;. As &sub2; character entity is non-existent, the string will appear as is, creating an impression that raw method somehow doesn't work.
Try to use ~2~mystring and it will be replaced with the superscript character entity ² and will be rendered correctly. This illustrates that your code correct, but the basic assumption about character entities is not.