re = /customers(?=(?:[^><]*<[^<>]*>)*[^><]*$)/m
str = '<p class="Pa0" style="margin: 0in 0in 0pt;"><span class="A1"><span style="font-size: 10pt;">Giving this flyer to your class customers</span></span></p>
'
subst = '<span style=\'color: red;\'>class<span>'
result = str.gsub(re, subst)
# Print the result of the substitution
puts result
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Ruby, please visit: http://ruby-doc.org/core-2.2.0/Regexp.html