It'd be extremely helpful if you mentioned how you want to split the Strings into substrings (e.g., do you want them to be split around spaces? Around a particular letter? Or what?)

If you read the String class documentation you'll see a method called split() that takes a regular expression as an argument; the method splits the String around matches of the regular expression. So if you were to create a regular expression that matched against the letter "K", then passed that into myString.split(regexForK), then that method call would return a String[], which is exactly what you want.

Using the pattern (\w+?[RK]) with the regex Matcher.find() method will capture all of those groups except for the last one, which doesn't end with R or K.

Edit: This pattern \w+?[RK]|\G\w++$ seems to produce the result you wanted.

Java Syntax (Toggle Plain Text)

Pattern pat = Pattern.compile("\\w+?[RK]|\\G\\w++$");
String input = "AMLARMLAKFGFP";
Matcher matcher = pat.matcher(input);
while (matcher.find()){
    System.out.println(matcher.group());
}

Pattern pat = Pattern.compile("\\w+?[RK]|\\G\\w++$"); String input = "AMLARMLAKFGFP"; Matcher matcher = pat.matcher(input); while (matcher.find()){ System.out.println(matcher.group()); }