I tried to solve the problem below using regular expression, but couldn't (end up using other approach): 

<http://community.topcoder.com/stat?c=problem_statement&pm=13006&rd=15846>

Given a string, you must check if contains the characters `C`, `A` and `T` in this order, and that there's only one occurrence of each. It can be other characters between these characters.
Ex:

```
CTA -> false
XCYAZTX -> true
ACBBAT -> false
CATT -> false
CCAT -> false
CAAT -> false
CAT -> true

```

So, it would be:

```
public String getAnswer(String S) {
   if(S.matches("pattern???")) return "Possible";
   return "Impossible";
}

```

Does someone know a pattern to use and can share please?