How to make a regular expression match based on a condition?

Question

I\'m trying to make a conditional regex, I know that there are other posts on stack overflow but there too specific to the problem.

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The Question

How can

Answer 1

I do not see the "conditional" in the question. The problem is solvable with a straight forward regular expression: \b\d+\b.

regex101 demo

A fully fledged Java example would look something like this:

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

class Ideone {
    public static void main(String args[]) {
        final String sample = "123 45 678 90";
        final Pattern pattern = Pattern.compile("\\b\\d+\\b");
        final Matcher matcher = pattern.matcher(sample);
        final ArrayList results = new ArrayList<>();
        while (matcher.find()) {
            results.add(matcher.group());
        }
        System.out.println(results);
    }
}

Output: [123, 45, 678, 90]

Ideone demo