regular expression validator

Try this:

string input = "R12036580479";

if(Regex.IsMatch(input, @"^[A-Z]\d{11}$"))
{
    Console.Write("It's a match");
}

Study this site carefully, you will know how to write Regular Expresssion.
Regular Expression Tutorial[^]

best of luck.

Get appropriate regular expression as you require from following link:
Expressions[^]

Similar answer:
Regular expression to validate a string[^]
Restrict user[^]

And a CP Article by Rahul:
A Tiny Javascript Framework for Common Validation Scenarios.[^]

Try:

[A-Z][0-9]{11}

Get a copy of Expresso [^] - it's free, and it examines and generates Regular expressions.

[edit]Typo - removed "," from regex - OriginalGriff[/edit]