Split repeated string by regex java

try this regex:

: (.*?)(?:Paper size|$) //global

regex demo output:

If your values can have any values, you may use a workaround splitting:

String s = "Paper size: A4Paper size: A3Paper size: A2";
String[] res = s.replaceFirst("^Paper size:\\s*", "") // Remove the first delimiter to get rid of the empty value
                .split("Paper size:\\s*");            // Split
System.out.println(Arrays.toString(res)); // => [A4, A3, A2]

See an IDEONE demo

Or, you can match any text other than Paper size: and capture it with ([^P]*(?:P(?!aper size:)[^P]*)*):

String s = "Paper size: A4Paper size: A3Paper size: A2";
String pattern1 = "Paper size: ([^P]*(?:P(?!aper size:)[^P]*)*)";
Pattern ptrn = Pattern.compile(pattern1);
Matcher matcher = ptrn.matcher(s);
List<String> res = new ArrayList<>();
while (matcher.find())
    res.add(matcher.group(1));
System.out.println(res); // => [A4, A3, A2]

See another IDEONE demo

The Paper size: ([^P]*(?:P(?!aper size:)[^P]*)*) is actually the same pattern as (?s)Paper size: (.*?)(?=Paper size: |\z), but an unrolled one, a much more efficient with longer inputs.

  String s = "Paper size: A4Paper size: A3Paper size: A2";
  String regex = "([A-Z]\\d)";

  Pattern p = Pattern.compile(regex);
  Matcher m = p.matcher(s);

  while(m.find()){
     System.out.println( m.group(1));
  }

Just simply replace your regex with this one : "Paper size: (..)?"

OR

"Paper size: (\\w\\d)?"

if you wanted to be strict that the captured group always consists of a letter followed by a number.

If you just need to get Paper size: A4 Paper size: A3 Paper size: A2 use String regex = "Paper size: A[234]";

System.out.println(m.group());