Asp.net (C #) code that limits user input of specified characters and numbers

Here is Code :
Only user names can be entered: the user name must start with 0 ~ 9. ~ Z or ~ Z! Copy code The Code is as follows: protected void button3_click (Object sender, eventargs E)
{
Int error_count = 0; // used to identify the validity of a user name
String STR = textbox1.text. Trim ();
If (STR = string. Empty)
{
Response. Write ("the user name cannot be blank! ");
Return;
}
STR = Str. substring (0, 1); // the user name contains 1st characters
String strchar = ", A, B, C, D, E, F, G, H, I, J, K, L, M, n, O, P, Q, R, S, T, U, V, W, X, Y, Z, A, B, C, D, E, F, G, h, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z ";
String [] vcarray = strchar. Split (',');
For (INT I = 0; I <vcarray. length; I ++)
{
If (STR! = Vcarray [I])
{
Error_count ++;
}
Else
{
Error_count = 0; // if the user name is valid, initialize the variable error_count to 0,
Break;
}
}
If (error_count> 0) // If the variable error_count is greater than 0, the user name is invalid.
{
Response. Write ("<SCRIPT> alert ('user name must start with 0 ~ 9. ~ Z or ~ Z! ') </SCRIPT> ");
}
Else
{
Response. Write ("<SCRIPT> alert ('legal user name, available! ') </SCRIPT> ");
}
}