Regex Markdown Header - ruby

Question

I'm trying to create a regular (ruby) expression which checks for multiple conditions. I use this regex to replace the content of my object. My regex is close to finished, except two problems I'm facing with regard to markdown.
First of, headers are giving me trouble. For example, I don't want to replace the word "Hi" for "Hello" if "Hi" is in a header.
Hi John <== # should not change
==================
Text: Hi, how are you? <== # Should be: Hello, how are you? after substitution
Or:
#### Hi Peter <== # should not change
Text: Hi, how are you? <== # Should be: Hello, how are you? after substitution
Question: How can I escape markdown headers within my regex? I've tried negative lookbehind and lookahead assertions, but to no avail.
My second problem should be quite easy, but somehow I'm struggling. If words are Italic "hi" I want to find and replace them, without changing the underscores. I can find the word with this regex:
\b[_]*hi[_]*\b
Question 2: But if I would replace it, I would also change the underscores. Is there a way to only detect the word itself and replace it, while still using word boundaries?
Code Example
#website.autolinks.all.each do |autolink|
autolink.name #for example returns "Iphone5"
autolink.url #for example returns "http://www.apple.com"
regex = /\b(?<!##\s)(?<![\d.\[])([_]*)#{autolink.name}([_]*)(?![\d'"<\/a>])\b/
if #permalink.blog_entry.content.match(regex)
#permalink.blog_entry.content.gsub!(regex, "[#{autolink.name}](# {autolink.url})")
end
end
Example text
Iphone5
==============
Iphone5 is the best mobile phone there is, even though the people at Samsung probably think, or perhaps only hope that their Samsung Galaxy S3 is better.
#### Samsung Galaxy S3?
Yes, that's the name of the newest Samsung phone.
This will result in a text with HTML tags, but when I use my regex my content uses Markdown syntax (used before the markdown converter).

Answer 1

Regexes work best when they do one clear thing. If you have multiple conditions, your code should usually reflect that by dividing the processing into steps.
In this case, you have two clear steps:
Use a simple regex or other logic to skip over the header portion of the message.
Once you know you are in the content, use another regex to process the content.

Answer 2

I've found a solution:
regex = /(?<!##\s)(?<![\d.\[a-z])#{autolink.name}(?![\d'"a-z<\/a>])(?!.*\n(==|--))/i
if #permalink.blog_entry.content.match(regex)
#permalink.blog_entry.content.gsub!(regex, "[\\0](#{autolink.url})")
end