import java.util.*;
import java.util.regex.*;
 
 
class rTest {
  public static void main (String[] args) {
 
String s = "I have been stuck on this problem forever quick brown fox jumps over";
String r = s.replaceAll("(?<=(?:^|\\G)\\S*\\s\\S*\\s\\S*)\\s", "_");
System.out.println(r);
 
String x = "I have been stuck on this problem forever quick brown fox jumps over";
String m = x.replaceAll("(?<=(^|\\G)\\S{0,100}\\s\\S{0,100}\\s\\S{0,100})\\s", "_");
System.out.println(m);
 
  }
}

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