Why is there an extra blank line after using gsub on file in Ruby with Thor?

I have some Ruby code that processes a Gemfile. It adds some recommended gems and removes other gems. There's a section of a Gemfile that looks like the following:

group :development, :test do
  # The gem version is a recommended starting place; upgrade if needed.
  gem 'pry-rails', '~> 0.3.4'
  # Enhance pry with byebug (which gives more debugger commands and other goodies).
  # The gem version is a recommended starting place; upgrade if needed.
  gem 'pry-byebug', '~> 3.4.0'
  # Use rspec for testing
  gem 'rspec-rails', '~> 3.5.1'
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
  gem 'byebug', platform: mri
end

I'm using the following two lines to remove the last two lines of the block (the gem 'byebug ... line and the comment above it).

gsub_file(full_app_gemfile_path, /^\s*gem\s*("|')byebug.*$/, "", verbose: false)
gsub_file(full_app_gemfile_path, /^\s*#.*Call.*("|')byebug("|').*$/, "", verbose: false)

The gsub_file is a method provided by the Thor gem. The removal works, but I end up with the following code in the Gemfile

group :development, :test do

  # The gem version is a recommended starting place; upgrade if needed.
  gem 'pry-rails', '~> 0.3.4'
  # Enhance pry with byebug (which gives more debugger commands and other goodies).
  # The gem version is a recommended starting place; upgrade if needed.
  gem 'pry-byebug', '~> 3.4.0'
  # Use rspec for testing
  gem 'rspec-rails', '~> 3.5.1'
end

Why is that extra blank line being inserted after group :development, :test do? It's nowhere near where the lines were removed. It could possibly be a bug in the Thor gem, but I'm wondering if it's regex issue.

Update

I just tried using the raw ruby gsub (to eliminate potential Thor issues). I created a helper method

def my_gsub(path, regex, str)
  text = File.read(path)
  rep = text.gsub(regex, str)
  File.open(path, "w") {|file| file.puts rep}
end

When I change the two lines that are calling gsub_file to call my_gsub, I now get two blank lines after group :development, :test do.

Solution

For your method my_gsub you are replacing the content of the line without replacing the linebreak. To make it remove the newline as well you can change your regex to this:

gsub_file(full_app_gemfile_path, /^\s*gem\s*("|')byebug.*$\n/, "")
gsub_file(full_app_gemfile_path, /^\s*#.*Call.*("|')byebug("|').*$\n/, "")