class String
  def starts_with?(str)
    return false if self.length < str.length
    return true if self[0, str.length] == str
    return false
  end

  def ends_with?(str)
    return self.reverse.starts_with?(str.reverse)
  end
end

Example:
"hello".starts_with?("hell")	 # => Returns true
"hello world!".ends_with?("world")	  # => Returns false (because it has an exclamation point, for which I'm not checking)