Regex Groups and Named Groups

Finding a date like 12-11-2026 is useful, but usually you want its parts: the day, the month and the year, separately. Wrapping parts of a pattern in parentheses creates groups, and the matcher remembers what each group matched. You can then read those parts by number or by name, or rearrange them in a replacement, for example to turn 12-11-2026 into 2026/11/12.

Numbered Groups

import java.util.regex.Matcher;
import java.util.regex.Pattern;
 
public class Dates {
    public static void main(String[] args) {
        Pattern date = Pattern.compile("(\\d{2})-(\\d{2})-(\\d{4})");
        Matcher m = date.matcher("Joined on 12-11-2026, renewed 05-01-2027");
 
        while (m.find()) {
            System.out.println("day " + m.group(1) + ", month " + m.group(2)
                + ", year " + m.group(3) + " (whole: " + m.group() + ")");
        }
    }
}

day 12, month 11, year 2026 (whole: 12-11-2026)
day 05, month 01, year 2027 (whole: 05-01-2027)

How it works

1. Each ( ... ) is a group, numbered 1, 2, 3 in the order their opening brackets appear.
2. group(1) returns what group 1 matched in the current match: the day.
3. group() (or group(0)) is the whole match.

Named Groups

Numbers become confusing in long patterns. Named groups, written (?<name> ... ), let you ask for parts by name:

import java.util.regex.Matcher;
import java.util.regex.Pattern;
 
public class Named {
    public static void main(String[] args) {
        Pattern date = Pattern.compile(
            "(?<day>\\d{2})-(?<month>\\d{2})-(?<year>\\d{4})");
        Matcher m = date.matcher("12-11-2026");
        if (m.matches()) {
            System.out.println(m.group("year") + "/" + m.group("month")
                + "/" + m.group("day"));
        }
 
        System.out.println("12-11-2026".replaceAll(
            "(\\d{2})-(\\d{2})-(\\d{4})", "$3/$2/$1"));
        System.out.println("12-11-2026".replaceAll(
            "(?<d>\\d{2})-(?<m>\\d{2})-(?<y>\\d{4})", "${y}/${m}/${d}"));
    }
}

2026/11/12
2026/11/12
2026/11/12

How it works

1. m.group("year") reads a named group; the code says what it means.
2. In a replacement, $1, $2, $3 insert numbered groups, so the date's parts can be rearranged.
3. Named groups are inserted with ${name}.

Non-Capturing Groups and Back-References

Sometimes you need brackets only to group alternatives, not to capture them. (?: ... ) groups without capturing, so it doesn't use up a number:

import java.util.regex.Matcher;
import java.util.regex.Pattern;
 
public class MoreGroups {
    public static void main(String[] args) {
        Matcher title = Pattern.compile("(?:Mr|Ms|Dr)\\. (\\w+)")
            .matcher("Dr. Advitha");
        if (title.matches()) {
            System.out.println(title.group(1)
                + ", groups: " + title.groupCount());
        }
 
        String typo = "this is is a test";
        System.out.println(typo.replaceAll("\\b(\\w+) \\1\\b", "$1"));
 
        Matcher weight = Pattern.compile("(\\d+)(kg)?").matcher("12");
        if (weight.matches()) {
            System.out.println("unit: " + weight.group(2));
        }
    }
}

Advitha, groups: 1
this is a test
unit: null

How it works

1. (?:Mr|Ms|Dr) groups the three titles for the | alternatives but doesn't capture, so the name is group 1, and there's only one group.
2. \1 inside a pattern is a back-reference: "the same text that group 1 matched". (\w+) \1 finds a word followed by itself, a repeated word, which the replacement fixes.
3. An optional group that didn't take part in the match returns null.

Common Mistakes

Counting groups by closing brackets. Number them by opening brackets, left to right.

Reading group(n) before calling find() or matches(). It throws IllegalStateException: No match found.

Not checking optional groups for null. A group that didn't participate returns null.

Practice

1. Easy: From "Navika Anamaneni", use two groups to capture the first name and surname, and print them as "Anamaneni, Navika" with replaceAll.
2. Harder: Parse "Rice x3 @ 54.50" with named groups item, qty and price, and print the line total.

Key Takeaways