Data driven framework under robot framework

Question

I am new to the Robot framework and am trying to write a testRobot.txt. I am using eclipse ide. If someone could help me out with a complete example I am trying to define the variables so that I can used them further inside the test case but every time it's showing "invalid variable name".

Code: *** Variables ***

 ${url}    https://application.com
 ${browser}      Chrome 
 ${username}     xpath://*[@id="content"]/app- 
 login/div[1]/div[2]/div[2]/div/form/div[1]/input
 ${password}     xpath://*[@id="content"]/app- 
 login/div[1]/div[2]/div[2]/div/form/div[2]/input
 ${login_button}  xpath://*[@id="content"]
 ${error}         Bad credentials

     Result:  



        Invalid variable name ' ${url}'.
        Invalid variable name ' ${browser}'.
        Invalid variable name ' ${username}'.
        Invalid variable name ' ${password}'.
        Invalid variable name ' ${login_button}'.
        Invalid variable name ' ${error}'.