Convert bunch of string to array and compare two arrays to get common values in both

This is my code -

 def self.constant_discovery(file_name, pattern) 
  matches = File.open(file_name).read.scan(pattern).uniq.flatten.compact
  matches_found = matches.map {|key| key.split(',')[0].scan(/^[A-Z]+(?:_[A-Z]+)*$/)}.flatten
  search_pattern = /([A-Z]+_)*[A-Z]+ = '.+'/
  File.open(file_name) do |f|
   lines = f.find_all {|line| line =~ search_pattern }
   lines.each do |line|
     if line.include?('=')
       print line unless line.empty?
       required_output = matches_found & line
     end 
    line
   end
 end
  matches 
end

By the above code I'm getting matches_found values like below, ["TEST_ONE", "TEST_TWO", "TEST_THREE","TEST_FOUR"]

Where as, line is producing output like this,

TEST_ONE = 'testone'
TEST_TWO = 'testtwo'
TEST_THREE = 'testthree'
TEST_FOUR = 'testfour'
KEY_AS_PRIMARY = 'primary'

In the require_output I need to get the common constant values from matches_found and line and from that common constant value I have to return the value which the common constant assigned to. While trying this I'm facing issue when comparing matches_found and line and getting error like "failed with error no implicit conversion of String into Array "

By this I understood that I have to convert values of line to an array and then should compare it with matches_found, after this I'll be getting common constant from both the arrays in required_output and later I can get the value which is assigned to those common constants as return values of that method. As I'm very new to ruby can someone please help me with it?