In case you will be working on a page that needs to validate the first character of the TextBox entered by a user then here are two options that you can use:
Option 1: Using an array
1: <asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" runat="server">
2: <script type="text/javascript">
3:
function CheckFirstChar(o) {
4: var arr = ['A', 'B', 'C', 'D'];
5: if (o.value.length > 0) {
6: for (var i = 0; i < arr.length; i++) {
7: if (o.value.charAt(0) == arr[i]) {
8: alert('Valid');
9: return true;
10: }
11: else {
12: alert('InValid');
13: return false;
14: }
15: }
16: }
17: }
18: </script>
19: </asp:Content>
20: <asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
21: <asp:TextBox ID="TextBox1" runat="server" onblur="return CheckFirstChar(this);"></asp:TextBox>
22: </asp:Content>
The example above uses an array of string for storing the list of characters that a TextBox value should start with. We then iterate to the array and compare the first character of TextBox value to see if it matches any characters from the array.
Option 2: Using Regular Expression (Preferred way)
1: <asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" runat="server">
2: <script type="text/javascript">
3:
function CheckFirstChar(o) {
4: pattern = /^(A|B|C|D)/;
5: if (!pattern.test(o.value)) {
6: alert('InValid');
7: return false;
8: } else {
9: alert('Valid');
10: return true;
11: }
12: }
13: </script>
14: </asp:Content>
15: <asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
16: <asp:TextBox ID="TextBox1" runat="server" onblur="return CheckFirstChar(this);"></asp:TextBox>
17: </asp:Content>
The example above uses regular expression with the pattern /^(A|B|C|D)/. This will check if the TextBox value starts with A,B,C or D. Please note that it's case sensitive. If you want to allow lower case then you can alter the patter to this /^(A|B|C|D)/i. The i in the last part will cause a case-insensitive search.
That's it! I hope someone find this post useful!
