Question

I have following Java code:

String s2 = "SUM   12  32 42";
Pattern pat1 = Pattern.compile("(PROD)|(SUM)(\\s+(\\d+))+");
Matcher m = pat1.matcher(s2);
System.out.println(m.matches());
System.out.println(m.groupCount());
for (int i = 1; i <=  m.groupCount(); ++i) {
        System.out.println(m.group(i));
}

which produces:

true
4
null
SUM
 42
42

I wonder what’s a null and why 12 and 32 are missing (I expected to find them amongst groups).

Answer 1

Group X from this part of your regex:

(\\s+(\\d+))+
|          |
+----------+--> X

will first match 12, then 32 and finally 42. Each time X‘s value gets changed, and replaces the previous one. If you want all values, you’ll need a Pattern & Matcher.find() approach:

String s = "SUM   12  32 42 PROD 1 2";
Matcher m = Pattern.compile("(PROD|SUM)((\\s+\\d+)+)").matcher(s);
while(m.find()) {
    System.out.println("Matched : " + m.group(1));
    Matcher values = Pattern.compile("\\d+").matcher(m.group(2));
    while(values.find()) {
        System.out.println("        : " + values.group());
    }
}

which will print:

Matched : SUM
        : 12
        : 32
        : 42
Matched : PROD
        : 1
        : 2

And you see a null printed because in group 1, there’s PROD, which you didn’t match.