Ruby add subdomain to URL

index.rb

# WAY 1
url = 'https://www.example.com/post/hello-world'
subdomain = 'blog'

index = url.index('://www.') ? url.index('://www.') + 7 : url.index('://') + 3
url.insert(index, subdomain + '.')
p url

# WAY 2
url = 'https://www.example.com/post/hello-world'
subdomain = 'blog'

address = url.include?('://www.') ? url.partition('://www.').last : url.partition('://').last
url.gsub!(address, subdomain + '.' + address)
p url

# WAY 3
url = 'https://www.example.com/post/hello-world'
subdomain = 'blog'

url.include?('://www.') ? url.gsub!('://www.', "://www.#{subdomain}.") : url.gsub!('://', "://#{subdomain}.")
p url