Thank you for all your help guys! Of course you are right, the simplest code ist the best for the given problem.
But I wanted to figure out how to solve it with an regular expression. Thank you Ireneusz Kordal for your correct regular expression!
The following code :
Delivers the following answer I needed:
This program delivers false when a String contains the String "TEST" and true otherwise (It is forbidden to have the Substring "TEST" in a String)
OK! String "TEST" delivers false
OK! String "TEST TEST" delivers false
OK! String "123 TEST 456" delivers false
OK! String "Peter" delivers true