How to use Pattern, Matcher in Java regex API to remove a specific line

Question

I have a complicate string split, I need to remove the comments , spaces , and keep all the numbers but change all string into character . If the - sign is at the start and followed by a number, treat it as a negative number rather than a operator

the comment has the style of ?<space>comments<space>? (the comments is a place holder)

Input :

-122+2 ? comments ?sa b
-122+2 ? blabla ?sa b

output :

 ["-122","+","2","?","s","a","b"]  

(all string into character and no space, no comments)

Answer 1

1. Replace the unwanted string \\s*\\?\\s*\\w+\\s*(?=\\?) with "" . You can chain String#replaceAll to remove any remaining whitespace. Note that ?= means positive lookahead and here it means \\s*\\?\\s*\\w+\\s* followed by a ? . I hope you already know that \\s specifies whitespace and \\w specifies a word character.
2. Then you can use the regex, ^-\\d+|\\d+|\\D which means either negative integer in the beginning (ie ^-\\d+ ) or digits (ie \\d+ ) or a non-digit ( \\D ).

Demo:

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Main {
    public static void main(String[] args) {
        String str = "-122+2 ? comments ?sa b";
        str = str.replaceAll("\\s*\\?\\s*\\w+\\s*(?=\\?)", "").replaceAll("\\s+", "");

        Pattern pattern = Pattern.compile("^-\\d+|\\d+|\\D");
        Matcher matcher = pattern.matcher(str);
        while (matcher.find()) {
            System.out.println(matcher.group());
        }
    }
}

Output:

-122
+
2
?
s
a
b