Regex to return all subdomains from a given domain

user_17335839 0
Viewed 57

Given a domain string like aaaa.bbbb.cccc.dddd I am trying to iterate over all of its subdomains i.e.

aaaa.bbbb.cccc.dddd
bbbb.cccc.dddd
cccc.dddd
dddd

I thought this regex ((?:[a-zA-Z0-9]+\.)*)([a-zA-Z0-9]+)$ should do the trick (please ignore the fact, that I am only matching these characters [a-zA-Z0-9]), however it only matches the full string.

How can I modify it to make it work?

Edit 1: The following code

var pattern = Pattern.compile("((?:[a-zA-Z0-9]+\\.)*)([a-zA-Z0-9]+)$"); //fixed regex here
var matcher = pattern.matcher("aaaa.bbbb.cccc.dddd");
matcher.results()
    .forEach(matchResult -> System.out.println(matchResult.group()));

should print (in any order)

aaaa.bbbb.cccc.dddd
bbbb.cccc.dddd
cccc.dddd
dddd
regex java
2 Answers

The regex you're looking for is

(?=(?:^|\.)([\.\w]+)*)

This pattern is based on lookahead. It can cross-match substrings that have already been matched in previous iterations.

Java Example

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Main {
    public static void main(String[] args) {
        final String regex = "(?=(?:^|\\.)([\\.\\w]+)*)";
        final String domain = "aaaa.bbbb.cccc.dddd";
        
        final Pattern pattern = Pattern.compile(regex);
        final Matcher matcher = pattern.matcher(domain);
        
        while (matcher.find()) {
            for (int i = 1; i <= matcher.groupCount(); i++) {
                System.out.println(matcher.group(i));
            }
        }
    }
}
user_127551870

This should group your blocks right: (([a-zA-Z].[a-zA-Z]*))

The output is following:

aaaa
aaaa
aaaa
bbbb
bbbb
bbbb
cccc
cccc
cccc
dddd
dddd
dddd

Now you can write your code and terminate the entries you did not need! This would be my fast solution.

user_197156970
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