def geekify(string)
  string.gsub(/[leto]/, 'l' => '1', 'e' => '3', 't' => '7', 'o' => '0')
end

geekify('leet') # => '1337'
geekify('noob') # => 'n00b'

def doctorize(string)
  string.gsub(/M(iste)?r/, 'Mister' => 'Doctor', 'Mr' => 'Dr')
end

doctorize('Mister Freeze') # => 'Doctor Freeze'
doctorize('Mr Smith')   # => 'Dr Smith'