Pattern/ Matcher Java, non zero group count but error retrieving?

Question

My matcher.groupCount() is giving me 4 but when I use matcher.group(0), ..., matcher.group(0), it gives me an error.

Following is my code:

Pattern pattern = Pattern.compile("([0-9]+).([0-9]+).([0-9]+).([0-9]+)");
Matcher matcher1, matcher2;

GeoIP[0][0] = (GeoIP[0][0]).trim();
GeoIP[0][1] = (GeoIP[0][1]).trim();

System.out.println(GeoIP[0][0]);
System.out.println(GeoIP[0][1]);

matcher1 = pattern.matcher(GeoIP[0][0]);
matcher2 = pattern.matcher(GeoIP[0][1]);

System.out.println("matcher1.groupCount() = " + matcher1.groupCount());
System.out.println("matcher2.groupCount() = " + matcher2.groupCount());

System.out.println("matcher1.group(0) = " (matcher1.group(0)).toString());

Console:

Exception in thread "main" 1.0.0.0
1.0.0.255
matcher1.groupCount() = 4
matcher2.groupCount() = 4

java.lang.IllegalStateException: No match found
    at java.util.regex.Matcher.group(Unknown Source)
    at filename.main(filename.java:linenumber)

the line number is pointing to

System.out.println("matcher1.group(0) = " (matcher1.group(0)).toString());

Answer 1

groupCount just tells you how many groups are defined in the regular expression. If you want to actually access a result, you have to perform a match first!

  if (matcher1.find()) {
    System.out.println("matcher1.group(0) = " (matcher1.group(0)).toString());
  } else {
    System.out.println("No match.");
  }

Also . is a special character in regex, you probably wanted \\.?

Answer 2

if I understand it right, you need access to the four bytes that create IP address. Instead of using groups you can try regex that match IP address and then split the found string.

String GeoIPs = "192.168.1.21, 10.16.254.1, 233.255.255.255";
Pattern pattern = Pattern.compile("\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}");
Matcher matcher;

matcher = pattern.matcher(GeoIPs);

while (matcher.find()) {
    String match = matcher.group();
    String[] ipParts = match.split("\\.");
    for (String part : ipParts) {
        System.out.print(part + "\t");
    }
    System.out.println();
}

There are some answers regarding Java regex for IP extracting: Extract ip addresses from Strings using regex and regex ip address from string.