我想检查输入的字符串是否格式正确

Question

I want to check the input string and validate if it is a correct number format and have written the following regex. [+-]?[0-9]*[.]?[0-9]*[e]?[+-]?[0-9]+. but unfortunately it is outputting true for --6 or ++6

import java.util.*;

public class Main {
    public static void main(String args[]) throws Exception {
        //Scanner
        Scanner in = new Scanner(System.in);
        int t = in.nextInt();
        in.nextLine();
        while(--t >= 0) {
            String string = in.nextLine();
            string = string.trim();
            //System.out.println(string);
            String regex = "[+-]?[0-9]*[.]?[0-9]*[e]?[+-]?[0-9]+";
            //System.out.println(string.matches(regex));
            if(string.matches(regex)) {
                System.out.println(1);
            }
            else {
                System.out.println(0);
            }
        }
    }
}

Answer 1

This is matching due to the second [+-]? in your regex string. In ++6 or --6 , it first matches the first +- since it is present, then again match the second +- since it was present, and then the digit.

But you were close. What you want to do is only match the second [+-]? if there is an exponent present. So just make the whole exponent part optional by enclosing in brackets and adding a ? at the end. Like this, you will match the second +- only if there is an e/E in front of it.

^[+-]?([0-9]+)?[.]?[0-9]*([eE][+-]?[0-9]+)?$

Regex Demo .

Answer 2

I would use this

^[+-]?(?:\\d+(?:\\.\\d*)?|\\.\\d+)(?:[eE][+-]?\\d+)?$

Formatted

 ^ 
 [+-]? 
 (?:
      \d+ 
      (?: \. \d* )?
   |  
      \. \d+ 
 )
 (?: [eE] [+-]? \d+ )?
 $