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
	{
		String regexSearch = "\\[[^\\]\\[|]*]";
		String regexReplacement = "*$0*";
		String inputString = "This is a User, [USER 1], and a second user [USER 2] not [USER | 3]";
		Pattern p = Pattern.compile(regexSearch);
		Matcher m = p.matcher(inputString);
		System.out.println(m.replaceAll(regexReplacement));
	}
}

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