Java Regex behaviour for Alternation (|) and + in a single expression

Question

Can some explain the following output for java regex:(CASE 1)

   String s = "topcoder is "

            String p = "(top|coder)+";
            Pattern pttrn = Pattern.compile(p);
            Matcher m = pttrn.matcher(s);
            while(m.find()){
                System.out.println("group count: " + m.groupCount());
                for (int i = 1; i <= m.groupCount(); i++) {
                    System.out.println("Found : \"" + m.group(i) + "\" as group " + i);
                }
                System.out.println("Zeroth group:" + m.group(0));
            }

produces following output:

group count: 1

Found : "coder" as group 1

Zeroth group:topcoder

Where the following code: (CASE 2)

        String s = "topcoder is ";
        String p = "(top|coder)";
        Pattern pttrn = Pattern.compile(p);
        Matcher m = pttrn.matcher(s);
        while(m.find()){
            System.out.println("group count: " + m.groupCount());
            for (int i = 1; i <= m.groupCount(); i++) {
                System.out.println("Found : \"" + m.group(i) + "\" as group " + i);
            }
            System.out.println("Zeroth group:" + m.group(0));
        }

produces following output:

group count: 1

Found : "top" as group 1

Zeroth group:top

group count: 1

Found : "coder" as group 1

Zeroth group:coder

Why isn't there are match for top in CASE 1 ? How is + affecting the matching for alternation (|) ?

When you repeat a capture group with a quantifier, the previous capture is overwritten by the next. — Casimir et Hippolyte
– Casimir et Hippolyte, Commented Jun 6, 2015 at 21:55

Sotirios Delimanolis · Accepted Answer · 2015-06-06 21:54:35Z

2

When using a capturing group that repeats, ie. because of a quantifier, the Matcher will only capture the last match.

You can wrap the entire repeated capturing group in its own capturing group to extract each match

String p = "((top|coder)+)";

This is explained in regex101.

answered Jun 6, 2015 at 21:54

Sotirios Delimanolis

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