/* 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 text = "random_ 123 _text 456 random_text_end";
        		Pattern pattern = Pattern.compile("(\\d+)[^\\d]*$");
        		Matcher matcher = pattern.matcher(text);
        		//если цифр в строке нет, то будет null.
        		String result = matcher.find() ? matcher.group(1) : null; 
        		System.out.println(result);
	}
}

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