input: Write a function to replace all occurrences of a given string with an alternate string in an input string. The function should return the modified string.
  def replace_string(input_string, target_string, replacement_string):
      return input_string.replace(target_string, replacement_string)
  
  assert replace_string("Hello World!", "World", "Python") == "Hello Python!"
  assert replace_string("Hello World!", "o", "a") == "Hella Warld!"
  assert replace_string("My name is John.", "John", "Alice") == "My name is Alice."
  assert replace_string("", "anything", "something") == ""
  assert replace_string("Hello World!", "", "Python") == "Hello World!"
output: def replace_string(input_string, target_string, replacement_string):
      return input_string.replace(target_string, replacement_string)