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/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
   public static void main (String[] args) throws java.lang.Exception
   {
     String regex = "(\\s)(\\*[^*]+\\*)(\\s)";
     String s = "test string *67* **Hi**";
     System.out.println(s.replaceAll(regex,"$1hello$3"));
   }
}
Success #stdin #stdout 0.05s 4386816KB
stdin
Standard input is empty
stdout
test string hello **Hi**