/* package whatever; // don't place package name! */
 
import java.util.*;
import java.util.regex.*;
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 text = "Hi How areYouToday";
		Matcher m = Pattern.compile("(?<=([a-z\\s]|^))[A-Z]").matcher(text);
 
		StringBuffer sb = new StringBuffer();
		while (m.find()) {
			if (!m.group(1).trim().isEmpty()){
				m.appendReplacement(sb, "_"+m.group().toLowerCase());
			}else{
				m.appendReplacement(sb, m.group().toLowerCase());
			}
		}
		m.appendTail(sb);
 
		System.out.println(sb.toString()); //Hi How are_you_today
	}
}

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