С регулярками все еще достаточно сложно. Придумал вот так:
public class Solution {
public static void main(String[] args) {
try(BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))){
String file1path= reader.readLine();
String file2path= reader.readLine();
try(BufferedReader fileReader= new BufferedReader(new FileReader(file1path));
BufferedWriter writer = new BufferedWriter(new FileWriter(file2path))){
StringBuilder str= new StringBuilder();
while (fileReader.ready()){
str.append(" ").append(fileReader.readLine());
}
Pattern my_pattern = Pattern.compile("[^0-9 ]", Pattern.CASE_INSENSITIVE);
List<String> answers = new ArrayList<>();
for(String x:str.toString().split(" ")){
if(!my_pattern.matcher(x).find()){
answers.add(x+" ");
}
}
for(String r:answers){
writer.write(r);
}
}
}catch (IOException e){
e.printStackTrace();
}
}
}