Regex test for ^ and $.

regex.txt

$ irb
irb(main):001:0> "".match? /^/
=> true
irb(main):002:0> "".match? /$/
=> true
irb(main):003:0> "".match? /^$/
=> true

irb(main):017:0> code = <<RUBY
irb(main):018:0" [1, 2, 3].each do
irb(main):019:0"   puts 'Hello, world!'
irb(main):020:0" end
irb(main):021:0" RUBY
=> "[1, 2, 3].each do\n  puts 'Hello, world!'\nend\n"
irb(main):022:0> 
irb(main):023:0> # すべての行を# でコメントアウトする
=> nil
irb(main):024:0> puts code.gsub(/^/, '# ')
# [1, 2, 3].each do
#   puts 'Hello, world!'
# end
=> nil