/* package whatever; // don't place package name! */
 
import java.util.*;
import java.lang.*;
import java.io.*;
import java.util.regex.*;
/* 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 = "start maddel1 meddel2 middel3 tail";
		String pat = "(^start|(?!\\A)\\G)\\s+m[aei]ddel[0-9](?=(?:\\s+m[aei]ddel[0-9])*\\s+tail$)";
		System.out.println(s.replaceAll(pat, "$1 <middle>" )); 
	}
}

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