C# regex that should start with alpha numeric but not with space

Question

Hi ,

I am trying for regex that should take only AlphaNumeric as first character but when i go with [A-za-z0-9], it is also taking white space as valid, how to avoid space as first character in regex C#.

please help.

below are some valid and invalid conditions (ignore starting and ending " ).

Invalid scenarios:

"!test"
"@test "
" Test"

Valid scenarios:

Test@3
test!12a
7test#1
67899
Test

Thanks in advance :)

Regards,
Sravan kumar

Answer 1

Hi @sravankumar-3852,

Regular expressions can be written like this:

var pattern = "^[a-zA-Z0-9]\\S+$";  

Below is the demo I tested, you can refer to it.

    class Program  
    {  
        static void Main(string[] args)  
        {  
            var pattern = "^[a-zA-Z0-9]\\S+$";  
            Console.WriteLine("Invalid scenarios:");  
            Console.WriteLine("!test:{0}", Regex.Match("!test", pattern).Success);  
            Console.WriteLine("@Test:{0}", Regex.Match("@Test", pattern).Success);  
            Console.WriteLine(" Test:{0}", Regex.Match(" Test", pattern).Success);  
            Console.WriteLine("\n\nValid scenarios:");  
            Console.WriteLine("Test@3:{0}", Regex.Match("Test@3", pattern).Success);  
            Console.WriteLine("test!12a:{0}", Regex.Match("test!12a", pattern).Success);  
            Console.WriteLine("7test#1:{0}", Regex.Match("7test#1", pattern).Success);  
            Console.WriteLine("67899:{0}", Regex.Match("67899", pattern).Success);  
            Console.WriteLine("Test:{0}", Regex.Match("Test", pattern).Success);  
            Console.ReadLine();  
        }  
    }  

Here is the result.

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Best Regards,
YihuiSun

Answer 2

Try several expressions:

^[a-zA-Z0-9]

(?i)^[a-z0-9]

^(\p{L}|\p{N})