06. Extract Emails-Runtime error

Elena123456 235 Точки

06. Extract Emails-Runtime error

Can somebody please explain to me why do I get in Judge- "Unhandled exception. System.Text.RegularExpressions.RegexParseException: Invalid pattern '(?<=\s)(?<user>(?![\_])[A-za-z0-9]+(?:[-\.\_][A-za-z0-9]+)*)@(?<host>[a-zA-Z]+(?:[\-][a-zA-z]+)*(?:\.[a-zA-Z]+(?:[\-][a-zA-Z]+)*)*\.[a-z]+)' at offset 21. Unrecognized escape sequence \\_."

Althouth in https://regex101.com/ and in https://regexr.com/ my regex is working correctly- match the valid and doesn't match the invalid emails:

valid email: info@softunibulgaria.org, kiki@hot-mail.co.uk, no-reply@github.com,
s.peterson@mail.uu.net, info-bg@software-university.software.academy support@github.com

invalid email: --123@gmail.com, …@mail.bg, .info@info.info, _steve@yahoo.cn,
mike@helloworld, mike@.unknown.soft., s.johnson@invalid-.

https://softuni.bg/trainings/resources/officedocument/49595/regular-expressions-exercise-csharp-fundamentals-may-2020/2830

https://judge.softuni.bg/Contests/Compete/Index/1668#5

using System;
using System.Text.RegularExpressions;
namespace text
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            string input = Console.ReadLine();
            Regex regexForEmail=new Regex(@"(?<=\s)(?<user>(?![\_])[A-za-z0-9]+(?:[-\.\_][A-za-z0-9]+)*)@(?<host>[a-zA-Z]+(?:[\-][a-zA-z]+)*(?:\.[a-zA-Z]+(?:[\-][a-zA-Z]+)*)*\.[a-z]+)");
            MatchCollection matchesEmail = regexForEmail.Matches(input);
            if (regexForEmail.IsMatch(input))
            {
                foreach (Match match in matchesEmail)
                {
                    Console.Write(match.Groups["user"]);
                    Console.WriteLine(match.Groups["host"]);
                }
            }

        }
    }
}

Тагове:

24/11/2020 12:25:56 C# Fundamentals 24/11/2020 14:02:47

Axiomatik 2422 Точки

Best Answer

Regex is also not running in VS, not just judge. I used this simpler Regex for this problem.

Best,

using System;
using System.Text.RegularExpressions;

namespace extractEmails
{
    class Program
    {
        static void Main(string[] args)
        {
            string text = Console.ReadLine();
            string pattern = @"(^|(?<=\s))(([a-zA-Z0-9]+)([\.\-_]?)([A-Za-z0-9]+)(@)([a-zA-Z]+([\.\-][A-Za-z]+)+))(\b|(?=\s))";
            Regex regex = new Regex(pattern);
            MatchCollection matches = regex.Matches(text);
            if (regex.IsMatch(text))
            {
                foreach (Match email in matches)
                {
                    Console.WriteLine(email);
                }
            }
        }
    }
}

24/11/2020 13:00:10

Elena123456 235 Точки

And thanks to you I have founded the mistake in my regex and I have learnt something new. I'am very happy and I'am very greatfull about that. :)

Regex is not running in VS and in Monodevelop when in front of "_" we put escape symbol "\".

I have barrowed this part of your regex :

[\.\-_]

and have deleted my "[-\.\_]".

And in the beginning in my regex I have replaced "?![\_]" with "?![_]".

Regex regexForEmail=new Regex(@"(?<=\s)(?<user>(?![_])[A-za-z0-9]+(?:[\.\-_][A-za-z0-9]+)*)@(?<host>[a-zA-Z]+(?:[\-][a-zA-z]+)*(?:\.[a-zA-Z]+(?:[\-][a-zA-Z]+)*)*\.[a-z]+)") ;

And now my regex is also valid and Judge gave to my code 100/100. :) https://pastebin.com/r6cZbrz5

Your regex is better than mine and I going to take a better look at it when I go home after work.

All the best!

Eli

24/11/2020 14:00:36 24/11/2020 14:04:05

Elena123456 235 Точки

Axiomatic, hello again!

I have taken a better look at your regex. I can't understand the begining:

(^|(?<=\s)

Why have you used operator "or" at the begining in your regex? If you have time to help me understand it I would realy appreciate it.

I see that your regex match and this email adress: "elena@abv-bg". I have made my regex more complicated, because I have assumed that Judge would be looking for a "." at the end, but I have assumed wrong.

By the way from today I have a new laptop with Windows 10 and VisualStudio 2019. I rushed and bought it earlier than I planned, because you mentioned that there is a potential risk to start Advanced module working on Monodevelop.Thanks a lot! :)

24/11/2020 22:34:09

Axiomatik 2422 Точки

No idea anymore, I just used different types of Backreferences/Lookahead/LookBehind and regex combinations until I found something that worked. But the regex on the exam won't be that complex and this exercise was more like a bonus assignment. Good luck with your new laptop and hopefully no more problems with C# code that won't compile with MonoDevelop.

Best,

25/11/2020 07:20:32