function validateValue() 
        {
        
            // Defining Regular Expression Pattern
            // This Pattern is validate the value like "a00000"
        
            var re5digit = /^\a\d{5}$/ 
            if (document.form1.TextBox1.value.search(re5digit) == -1) //if match failed
                alert("Please enter a valid 5 digit number inside the text starts with 'a', Example: a00000")
        }