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Introduction

Regular Expressions are very much useful for validation checking. It's not a new technology; it originated in the UNIX environment, and is commonly used with the Perl language. Regular expressions are, however, supported by a number of .NET classes in the namespace System.Text.RegularExpressions.

Its rules are same as the finite automata. Information regarding the main special characters or escape sequences that you can use are available in the MSDN.

Regular Expressions for Email Checking

Basic things to be understood in RegEx are:

The rules for validating email IDs, and some valid and invalid examples are mentioned here:

  1. Email addresses must be start with a letter symbol. And any number of letters or digits or underscore (_) can be appended, and only a single dot (.) is allowed but other symbols and white spaces are not allowed.
  2. The name field of the address must end with either a letter or digit.
  3. If underscore or dote is used then before it, letters or digits must be used for a valid name.
  4. Dot can be used only once but underscore can be used multiple times.

Some examples:

miltoncse00@yahoo.com            valid
2milton00@yahoo.com              invalid
milton cse00@yahoo.com           invalid(white space)
milton_cse@yahoo.com             valid
milton_cse_00@yahoo.com          valid
milton_cse_00_@yahoo.com         invalid(_ before @)
milton.case_00_00@yahoo.com      valid

milton.cas.e_00_00@yahoo.com     invalid(double dote)
milton.cas.e_00_00@yahoo.co.in   valid
milton.cas.e_00_00.@yahoo.co.in  valid(dote before @)
miltoncse00@yahoo.com      
miltoncse00                      name portion

According to these rules and valid examples, we can draw a state diagram for valid name checking of email addresses:

Fig: state diagram for the naming portion

From the state diagram, the regular expression for the naming part is:

[a-z][a-z|0-9|]*([_][a-z|0-9]+)*([.][a-z|0-9]+([_][a-z|0-9]+)*)?

The rules for the email name portion (before @) can start with a letter. And any number of letters or digits can be appended and other symbols are not allowed.

So the regular expression for that part is:

[a-z][a-z|0-9|]*

After the dot (.) portion like (.com/.net), it can start with a letter and any number of letters or digits can be appended. If another dot portion is allowed then that can follow the same rule.

So the regular expression for that part is:

([a-z][a-z|0-9]*(\.[a-z][a-z|0-9]*)?)

Combining all these regular expression, the regular expression for email checking that satisfies the Yahoo! email rules will be:

^[a-z][a-z|0-9|]*([_][a-z|0-9]+)*([.][a-z|0-9]+([_][a-z|0-9]+)*)?
@[a-z][a-z|0-9|]*
\.([a-z][a-z|0-9]*(\.[a-z][a-z|0-9]*)?)$

The C# code that can find that matching is very simple, as illustrated bellow:

string pattern=@"^[a-z][a-z|0-9|]*([_][a-z|0-9]+)*([.][a-z|" + 
               @"0-9]+([_][a-z|0-9]+)*)?@[a-z][a-z|0-9|]*\.([a-z]" + 
               @"[a-z|0-9]*(\.[a-z][a-z|0-9]*)?)$";
System.Text.RegularExpressions.Match match = 
    Regex.Match(txtEmail.Text.Trim(), pattern, RegexOptions.IgnoreCase);

if(match.Success)
    MessageBox.Show("Success");
else
    MessageBox.Show("Fail");

So, we conclude that any validation problems that involve recursion, option, limitation is easier to solve with regular expressions than using other ways (like if-elseif-else, while condition). This can be represented in a state diagram that is very much easier and efficient to express and use.

My next article will be on auto ID generation for any table using stored procedures.

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GeneralPoor article. Example of validation in real world. Pin
shaoun1000		21:59 28 Dec '08  
GeneralRe: Poor article. Example of validation in real world. Pin
The JZ		4:12 6 Jan '09  
GeneralRe: Poor article. Example of validation in real world. Pin
milton cse00		23:57 8 Jan '09  
GeneralRe: Poor article. Example of validation in real world. Pin
shaoun1000		6:25 23 Jun '09  
GeneralNot working.... Pin
jasysam		23:33 24 Sep '08  
GeneralRe: Not working.... Pin
milton cse00		23:59 8 Jan '09  
GeneralHow do I validate internet (i.e Attach File path) Pin
Dhaval Pairkh		11:00 23 Jul '07  
Generalvalidation of email account Pin
QuickDeveloper		21:47 27 Apr '06  
GeneralRe: validation of email account Pin
Vasudevan Deepak Kumar		3:05 28 Apr '06  
GeneralRe: validation of email account Pin
milton cse00		20:00 14 May '07  
GeneralRe: validation of email account Pin
shaoun1000		19:04 16 Dec '08  
GeneralRe: validation of email account Pin
adima_76		4:45 19 Jun '07  
GeneralToo Simple Pin
RobertAn		13:52 27 Apr '06  
GeneralRe: Too Simple Pin
Jake Wharton		16:28 27 Apr '06  
GeneralRe: Too Simple Pin
chmod755		17:25 27 Apr '06  
GeneralRe: Too Simple Pin
jrandomuser		6:08 8 May '06  
GeneralRe: Too Simple Pin
Mark Cranness		20:50 15 May '06  
GeneralRe: Too Simple Pin
Cd-MaN		22:01 15 May '06  
GeneralRe: Too Simple Pin
winart		21:54 17 May '06  
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