/* 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 s = "I have been stuck on this problem forever quick brown fox jumps over";
		String r = s.replaceAll("(?<=(^|\\G)\\S{0,100}\\s\\S{0,100}\\s\\S{0,100})\\s", "_");
		System.out.println(r);
	}
}

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