Ruby Rubocop - Prefer the use of MatchData

Question

I inherited a Ruby on Rails project (v 2.0) from another developer and I am attempting to clean it up by running rubocop on it. For those who don't know, rubocop is just a tool used to ensure the code is standardized and clean throughout the entire project, catching little formatting errors and others as well. The function where I'm getting an offence at is shown below on the line 3 (data = $1):

def save_file(photo_data)
  photo_data =~ /data:image\/jpeg;base64,(.*)/
  data = $1
  File.open(raw_photo_location, 'wb') do |f|
    f.write(Base64.decode64(data))
  end
  return true
end

Rubocop's message on that line states: "Prefer the use of MatchData over $1."

I'm not sure how to effectively correct this preference issue because I am the least bit familiar with Ruby on Rails and their syntax & semantics. What I do know is that $1 is a global variable. Could anyone direct me on the formatting fix here? I appreciate anyone's help in advanced and I apologize if this question is really silly.

Answer 1

This should work:

def save_file(photo_data)
  data = /data:image\/jpeg;base64,(.*)/.match(photo_data)[1]
  File.open(raw_photo_location, 'wb') do |f|
    f.write(Base64.decode64(data))
  end
  return true
end

Answer 2

You shouldn't use =~ or .match for this.

The idiomatic way would be to use []:

data = photo_data[/data:image\/jpeg;base64,(.*)/, 1]