# Common Use Pattern: Length Validation

The primary application of OP\_SIZE is **enforcing data format requirements**—ensuring inputs meet expected size constraints before processing.

**Example: String length limit**

```
OP_SIZE OP_10 OP_LESSTHANOREQUAL OP_VERIFY
```

**How this works:**

1. **Measure top item:** OP\_SIZE determines byte length
2. **Compare to 10:** Check if length ≤ 10 bytes
3. **Enforce requirement:** OP\_VERIFY fails script if false

**Why this matters:**

* **Input sanitization:** Reject oversized data before processing
* **Protocol compliance:** Ensure data fits expected format
* **Resource protection:** Prevent excessive computation on huge inputs
* **User feedback:** Clear failure point for invalid submissions

 

**Real-world example:** A script accepting user-submitted usernames might enforce maximum length:

```
// Stack: <username>
OP_SIZE OP_20 OP_LESSTHANOREQUAL OP_VERIFY
// Continue with username validation
<rest_of_validation_logic>
```

This prevents someone from submitting a 1MB "username" that could cause processing issues.


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