---
title: How to validate a form field for a barcode format like ABCD0312
description: "This guide shows how to create a validation rule that ensures: The first 4 characters are uppercase letters (A–Z) and the last 4 characters are digits (0–9)"
---
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# How to validate a form field for a barcode format like ABCD0312
## This guide shows how to create a validation rule that ensures the first 4 characters are uppercase letters (A–Z) and the last 4 characters are digits (0–9).
#### **What you'll build**
A field called **Barcode** that only validates for a structure of 4 capital letters followed by 4 digits, e.g. `ABCD0312`.
| Input | Valid? |
| --- | --- |
| `ABCD0312` | ✅ Yes |
| `abcD0312` | ❌ No |
| `ABCD123` | ❌ No |
| `A1C20312` | ❌ No |
| `ABCD03123` | ❌ No |
#### **Step 1 — Open the field's validation rules**
In the **Barcode** field, navigate to the **Field Validation Rules** tab.
#### **Step 2 — Add the valid regex rule**
Under "Mark form as Valid without message, when":
```
data["Barcode"]=~"^[A-Z]{4}[0-9]{4}$"
```
This regex means:
- `^[A-Z]{4}` → Starts with 4 capital letters
- `[0-9]{4}$` → Ends with 4 digits
- `^...$` → Anchors it to **exactly 8 characters**, no more, no less
#### **Step 3 — Add the catch-all invalid rule**
Add a new validation rule with the `+` icon, click the "Valid" button to change it to "Invalid", and select a colour.
Under "Otherwise, mark form as Invalid without message, when":
```
1==1
```
This always evaluates as true, meaning any unmatched input is invalid by default.
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