Regex Validating CSV Email Addresses

Introduction

One of the most strengths added to Microsoft .NET framework is Regular expressions language, Actually I have passion to regular expressions, I used it intensively to build a meta search engine, and you can do more with less when you are using regular expressions (a.k.a.: Regex), most of times I find myself going to this solution even in non-technical situations; it helps so much to get away from string parsing.

Regex Tools and Resources

One of the earliest sites, that has been launched since .NET 1.0 is RegexLib , it's so helpful and it contains a repository of Regular expressions for any pattern you will ever need, and it has a test module to test your Regex against input text you supply.

Learning Regex is somehow painful, but once you get acquainted it will be a real fun!. so you will find so many tools helping you to build and test your own Regular expressions.

One of the earliest tools is Expresso, which started as a code project article then it has been so popular and usable by most of .NET Geeks.

Regulator, is one of the most cool tools, it has a builder like Expresso and also it has a great intellisense feature that allows you to remember Regex syntax easily, and imagine what it has a search engine that lets you search in RegexLib repository (actually it's a webservice call sent to RegexLib.com) so you are not asked to start building your Regex from scratch!.

Common feature between Expresso & Regulator both of them provides reverse text feature it means if you supply one Regex as : \d+ , it generates a description like : " matches any non-zero length digit " , ain't it cool !.

You will find so many articles about how to get your hands on Regular Expression Language, check these links.

Case Study

Problem

You have a comma separated group of email addresses and you want to make sure that each email address is valid one.

Design

Regular expression is the most adequate solution for such a case, so I will use regular expression to make sure each address is a valid one, so the first step is how to build this regular expression, a prerequisite is that you are familiar with regular expression as a language, otherwise I suggest that you search RegexLib.com.

Solution

Building Regular Expression

After some iterations with Expresso, I deducted the following:

Input Sample : kareem@yahoo.com,Mike@gmail.com,Michael@hotmail.com
Objective : making sure that each of these addresses is a valid one.
Regex : [a-zA-Z0-9_\-\.]+@[a-zA-Z0-9_\-\.]+\.[a-zA-Z]{2,5}

Let's have a look at this regular expression, I will follow divide and conquer to explain each piece of this regex

[a-zA-Z0-9_\-\.]+ : means any character lower or upper case, or any digit from 0 to 9 and also "_" is allowed, also "-" or ".", the + means this cannot be optional you should supply non-zero length string before the @ sign.

@ : each and every email address should have the @ sign.

[a-zA-Z0-9_\-\.]+ : explained above.

\. : It's the . that's followed by com, org, net, etc..

[a-zA-Z]{2,5} : this is the com, net, biz, etc.. , it's any character upper or lower case and the length should be between 2 and 5.

C# Code To Access Matched Addresses

You can access the matched email address using the following snippet

            Regex matchRegex = new Regex(@"[a-zA-Z0-9_\-\.]+@[a-zA-Z0-9_\-\.]+\.[a-zA-Z]{2,5}");            

            MatchCollection matches  =

                matchRegex.Matches("zako@hotmail.com, weko2@yhaoo.com,kareem@gmail.com");

            if ( matches.Count > 0) 

            {

                for ( int i = 0; i <>
 
                {
                    // You can process the email matched 
                    // for example you can check if it's a real email address or not.
                    // by pinging the mail provider.
                    ProcessEmailAddress(matches[i].Value);
                }
            }

Note: don't forget to add the inclusive directive using System.Text.RegularExpressions;