Help with Regular Expression for field validation

Question

I'm fairly new to using regular expressions and using Acrobat. This is probably a simple question, but I've been unable to figure it out.
I have a text field on a PDF that I would like to be 9 characters in length. The first 2 characters can only be alphanumeric, the last 7 characters can only be numeric.
At first I was using the following, which allows all the characters to be alphanumeric:
var re = /^[A-Za-z0-9 :\\_]$/;
if (event.change.length >0) {
if (event.willCommit == false) {
    if (!re.test(event.change)) {
        event.rc = false
That works fine, but it's not quite what I needed. With some assistance I changed it (see below) to fit what I was looking for. However, this didn't work; it prevents anything from being entered in the field:
var re = /^[A-Za-z0-9]{2}\d{7}$/;
if (event.change.length >0) {
if (event.willCommit == false) {
    if (!re.test(event.change)) {
        event.rc = false
Any help would be greatly appreciated.
Thanks...

Answer 1

Here's a function you can call form the field's custom Format script. It should be placed in a document-level JavaScript:
function custom_ks1() {
    // Define non-commited regular expression
    var re = /^[A-Za-z0-9]{0,2}([0-9]{0,7})?$/;
    // Get all of the characters the user has entered
    var value = AFMergeChange(event);
    // Allow field to be cleared
    if(!value) return;
    if (event.willCommit) {
        // Define commited regular expression
        var re = /^[A-Za-z0-9]{2}[0-9]{7}$/;
        if (!re.test(value)) {  // If final value doesn't match, alert user
            app.alert("Your error message goes here.");
            // event.rc = false
    } else {  // not commited
        // Only allow characters that match the regular expression
        event.rc = re.test(value);
Call it like this:
// Custom Keystroke script
custom1_ks();