demo, Java - rextester

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//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

import java.util.*;
import java.lang.*;
import java.util.regex.*;
import java.util.stream.*;
import java.util.stream.Collectors;

class Rextester
{
    public static String getCSV(int start, int end) {
        List<String> list = IntStream.range(start, end).boxed().map(i -> i.toString()).collect(Collectors.toList());
        String csv = String.join(" ", list);

return csv;
    }

public static void main(String args[])
    {
        String input = "1, 2, 3, 4.. 9, 10,13.. 17, 18, 19.. 25";
        input = input.replaceAll(",\\s*", " ");
        Pattern r = Pattern.compile("(\\d+)\\.\\.\\s*(\\d+)");
        Matcher m = r.matcher(input);
        StringBuffer stringBuffer = new StringBuffer();
        while (m.find( )) {
            int start = Integer.parseInt(m.group(1));
            int end = Integer.parseInt(m.group(2));
            m.appendReplacement(stringBuffer, getCSV(start, end+1));
            System.out.println("start: " + start + ", end: " + end);
            //String output = m.replaceFirst(getCSV(start, end));
            //System.out.println(output);
        }
        System.out.println(stringBuffer);
    }
}

[ + ] Show input
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