Regular expression to proof if a number is dividable by 5 in Java-CodePudding

So, I have to write an Regex statement which proofs if a number is dividable by 5 and returns either true or false. So, after some research I found out that any number which ends with a 0 or 5 is dividiable by 5 and therefore I try to match the last digit of the string with either a 0 or 5.

However, it does not seem to work as the method only returns false, although it worked on an online regex checker.

public class MyClass {
    public static void main(String args[]) {
      
     String[] digits = {"20", "25", "000", "001", "5123456789"};
        for (String s : digits){
            System.out.println("\""   s   "\""   " -> "   proof(s));
        }
    
    
    }
        
    public static boolean proof(String s){
        return s.matches("[0|5]$"); // Here is somehow an issue.
    }
}

CodePudding user response：

Here's one of the possible ways to implement this:

public static boolean proof(String s) {
    return s.matches("([1-9]\\d*)?[05]");
}

Where ([1-9]\\d*)? matches an optional group of characters (i.e. it might not be present) which should start with a digit from 1 to 9 followed by zero or more digits.

Note

Pipe symbol '|' is not needed. When you're using square brackets [], you only need to enumerate the symbols you need to match, i.e. [05].
While using matches() regular expression would be applied to the whole string, i.e. neither ^ and the very beginning, no $ at the very end are not needed.
Disclaimer: in my interpretation of the problem statement leading zeros are not allowed. If this requirement is not relevant, then the following regular expression would be sufficient: "\\d*[05]".

To avoid compiling this regular expression at each method call, we can extract it into static final field. And since you need a boolean result it would be convenient to keep it as a Predicate, for that we can make use of Java 11 Pattern.asMatchPredicate()

public static final Predicate<String> IS_DIVISIBLE_BY_FIVE =
    Pattern.compile("([1-9]\\d*)?[05]").asMatchPredicate();

public static boolean proof(String s) {
    return IS_DIVISIBLE_BY_FIVE.test(s);
}

main()

public static void main(String[] args) {
    String[] digits = {"20", "25", "000", "001", "5123456789"};
    
    for (String number : digits) {
        System.out.println(number   " -> "   proof(number));
    }
}

Output:

20 -> true
25 -> true
000 -> false
001 -> false
5123456789 -> false