Regular Expression for any number divisible by 60 using C# .Net?

Question

I need to apply validation on input time intervals that are taken in as seconds. Now i am not really good at Regular expressions. So can any body help making a regular expre

Answer 1

Note: Regular expressions are not the right tool for this. This is just for fun and to see how it could be done.

Here is a regular expression that tests if a number is divisible by 60:

^0$|^(?!0)(?=.*[02468]0$)(?:[0369]|[147](?:[0369]*[147][0369]*[258])*(?:[0369]*[258]|[0369]*[147][0369]*[147])|[258](?:[0369]*[258][0369]*[147])*(?:[0369]*[147]|[0369]*[258][0369]*[258]))*0$

It works by testing if the number is divisible by 3 and using a lookahead to check if it is divisble by 20. It differs from the regular expression posted by tiftik in the following ways:

• It is slightly shorter.
• It uses non-capturing groups.
• Many languages (for example Javascript )don't support variable length lookbehind assertions. This regular expression does not use lookbehinds so it can for example also be used for client-side validation in a web application.
• It disallows numbers with leading zeros (if you want to allow leading zeros just remove the (?!0)).

This is the code I used to generate and test it:

using System;
using System.Text;
using System.Text.RegularExpressions;

class Program
{
    Regex createRegex()
    {
        string a = "[0369]*";
        string b = "a[147]";
        string c = "a[258]";
        string r = "^0$|^(?!0)(?=.*[02468]0$)(?:[0369]|[147](?:bc)*(?:c|bb)|[258](?:cb)*(?:b|cc))*0$";
        r = r.Replace("b", b).Replace("c", c).Replace("a", a);
        return new Regex(r);
    }

    bool isDivisibleBy3(string s)
    {
        int sumOfDigits = 0;
        foreach (char c in s)
        {
            sumOfDigits += c - '0';
        }
        return sumOfDigits % 3 == 0;
    }

    bool isDivisibleBy20(string s)
    {
        return Regex.IsMatch(s, "^0$|[02468]0$");
    }

    bool isDivisibleBy60(string s)
    {
        return isDivisibleBy3(s) && isDivisibleBy20(s);
    }

    bool isValid(string s)
    {
        return Regex.IsMatch(s, "^0$|^[1-9][0-9]*$");
    }

    void Run()
    {
        Regex regex = createRegex();
        Console.WriteLine(regex);

        // Test on some random strings.
        Random random = new Random();
        for (int i = 0; i < 100000; ++i)
        {
            int length = random.Next(50);
            StringBuilder sb = new StringBuilder();
            for (int j = 0; j < length; ++j)
            {
                sb.Append(random.Next(10));
            }
            string s = sb.ToString();
            bool isMatch = regex.IsMatch(s);
            bool expected = isValid(s) && isDivisibleBy60(s);
            if (isMatch != expected)
            {
                Console.WriteLine("Failed for " + s);
            }
        }
    }

    static void Main()
    {
        new Program().Run();
    }
}