import java.util.*;
import java.lang.*;
import java.io.*;
import java.util.regex.*;
 
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		List<String> strs = Arrays.asList("xxx _italic some text 123 \\_*%&$ _ xxx",
                                          "\\_test_test_");
        String regex = "(?s)(?<!\\\\)(?:\\\\{2})*_((?:[^\\\\_]|\\\\.)+)_";
        Pattern p = Pattern.compile(regex);
		for (String str : strs) {
			Matcher m = p.matcher(str);
			List<String> result = new ArrayList<>();
			while(m.find()) {
				result.add(m.group(1));
			}
			System.out.println(str + " => " + String.join(", ", result));
		}
	}
}

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