regular exression for concatenating string in 2 different lines into the same line

hello ,

i have to extract the number present in the second line using regular expression . but am not able to do so tried with all type of regular expression using the search string as the first line since this was requirement given .
currently i am converting pdf to text and from this text i need to extract this number .
could anyone please help me out

the format in text file is like this below

Beleg-Nr.:
2001338138

but i am settingsearch string as beleg-nr....and then trying to extract next line which is not happening . Or is there a way out so that i can concatenate both and then extract the number from it

import java.nio.channels.*;
 import java.io.*;
import java.util.regex.*;
 
public class RECH {
    public static void main(String[] args) throws Exception {
      try
      {
		    String pattern = "Beleg-Nr.:[\\n].*";

BufferedReader bf = new BufferedReader(new FileReader("d:\\pdf\\pdf2\\output1.doc"));
			int linecount = 0;
			String line="";
			boolean isWordMatched = false;
			String value = "";
			String search = "Beleg-Nr.:";
            while ((line = bf.readLine()) != null) {
				if (line.indexOf(search) != -1) {
					System.out.println("Beleg-Nr.: found at  position"+line.indexOf(search));
                     Pattern p = Pattern.compile(pattern);
                      boolean found = p.matcher(line).lookingAt();
			       //System.out.println("'" + pattern + "'" + (found ? " matches '" : " doesn't match '") + line+ "'");
					System.out.println("line    "+line);  
 break;											} //if
	       
                                                                 } //while

}
      catch (Exception e)
      {
      }
	}
}

If the items are on separate lines might it not be easier to just read each line and discard the ones you don't need?