In this article I will explain with an example, how to implement GST Number validation using Regular Expression (Regex) in JavaScript.
HTML Markup
The following HTML Markup consists of an HTML TextBox, SPAN and a Button.
The Button has been assigned a JavaScript OnClick event handler.
GST Number:
<input type="text" id="txtGSTNumber" class="gst" />
<span id="lblError" class="error"></span>
<hr />
<input type="button" value="Submit" onclick="ValidateGSTNumber()" />
Validating GST Number using Regular Expression in JavaScript
When the Submit Button is clicked, the ValidateGSTNumber JavaScript function is executed.
Inside the ValidateGSTNumber JavaScript function, the GST Number TextBox is referenced and its value is tested using a Regular Expression.
The following conditions must satisfy for a GST Number to be termed as valid.
1. It should be 15 characters long.
2. The first 2 characters should be a digit representing State Code.
3. The next 10 characters should be PAN number of the taxpayer.
Note: For more details on PAN Card Number validation, please refer my article PAN Card Number validation using JavaScript and Regular Expression.
4. The 13th character should be any digit (1-9) or an alphabet.
5. The 14th character should be Z by default.
6. The 15th character should be an alphabet or any digit. 0-9.
Example: 06BZAHM6385P6Z2
<script type="text/javascript">
    function ValidateGSTNumber() {
        var gstNumber = document.getElementById("txtGSTNumber").value;
        var lblError = document.getElementById("lblError");
        lblError.innerHTML = "";
        var expr = /^[0-9]{2}[A-Z]{5}[0-9]{4}[A-Z]{1}[1-9A-Z]{1}Z[0-9A-Z]{1}$/;
        if (!expr.test(gstNumber)) {
            lblError.innerHTML = "Invalid GST Number.";
CSS Class
The following CSS classes are used.
1. error – It will apply RED color to the error message.
2. gst – It will force the letters to be UPPER case.
<style type="text/css">
    body { font-family: Arial; font-size: 10pt; }
    .error { color: Red; }
    .gst { text-transform: uppercase; }
GST (Goods and Services Tax) Number validation using JavaScript and Regular Expression
Browser Compatibility

The above code has been tested in the following browsers.

Internet Explorer  FireFox  Chrome  Safari  Opera 

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