i can have in textbox1:
A123
B899
G767
K990
..
8909<wrong value
I need before to click on button1 is present the firt char is letter in textbox1
note:
the value can have only 4 digit
A123
B899
G767
K990
..
8909<wrong value
I need before to click on button1 is present the firt char is letter in textbox1
note:
the value can have only 4 digit