# Character reference expands to a control character (U+0002).
> Canonical HTML version: https://rocketvalidator.com/html-validation/character-reference-expands-to-a-control-character-u-0002
> Attribution: Rocket Validator (https://rocketvalidator.com)
> License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)
## What Are Control Characters?
Control characters occupy code points U+0000 through U+001F and U+007F through U+009F in Unicode. They were originally designed for controlling hardware devices (e.g., U+0002 is "Start of Text," U+0007 is "Bell," U+001B is "Escape"). These characters have no visual representation and carry no semantic meaning in a web document.
The HTML specification explicitly forbids character references that resolve to most control characters. Even though the syntax `` is a structurally valid character reference, the character it points to is not a permissible content character. The W3C validator raises this error to flag references like `�`, ``, ``, ` `, and others that fall within the control character ranges.
## Why This Is a Problem
- **Standards compliance:** The [WHATWG HTML Living Standard](https://html.spec.whatwg.org/multipage/syntax.html#character-references) defines a specific set of "noncharacter" and "control character" code points that must not be referenced. Using them produces a parse error.
- **Unpredictable rendering:** Browsers handle illegal control characters inconsistently. Some may silently discard them, others may render a replacement character (�), and others may exhibit unexpected behavior.
- **Accessibility:** Screen readers and other assistive technologies may choke on or misinterpret control characters, degrading the experience for users who rely on these tools.
- **Data integrity:** Control characters in your markup often indicate a copy-paste error, a corrupted data source, or a templating bug that inserts raw binary data into HTML output.
## How to Fix It
1. **Identify the offending reference** — look for character references like ``, ``, `�`, ``, or similar that point to control character code points.
2. **Determine intent** — figure out what character or content was actually intended. Often, a control character reference is the result of a bug in a data pipeline or template engine.
3. **Remove or replace** — either delete the reference entirely or replace it with the correct printable character or HTML entity.
## Examples
### Incorrect: Control character reference
This markup contains ``, which expands to the control character U+0002 (Start of Text) and triggers the validation error:
```html
Some text more text
```
### Incorrect: Hexadecimal form of a control character
The same problem occurs with the hexadecimal syntax:
```html
Data:
```
### Correct: Remove the control character reference
If the control character was unintentional, simply remove it:
```html
Some text more text
```
### Correct: Use a valid character reference instead
If you intended to display a special character, use the correct printable code point or named entity. For example, to display a bullet (•), copyright sign (©), or ampersand (&):
```html
Item • Details
Copyright © 2024
Tom & Jerry
```
### Correct: Full document without control characters
```html
Example Page
This paragraph uses only valid character references: & < > ©
```
### Common Control Character Code Points to Avoid
| Reference | Code Point | Name |
|-----------|-----------|------|
| `�` | U+0000 | Null |
| `` | U+0001 | Start of Heading |
| `` | U+0002 | Start of Text |
| `` | U+0007 | Bell |
| `` | U+0008 | Backspace |
| ` ` | U+000B | Vertical Tab |
| `` | U+000C | Form Feed |
| `` | U+007F | Delete |
If your content is generated dynamically (from a database, API, or user input), sanitize the data before inserting it into HTML to strip out control characters. Most server-side languages and templating engines provide utilities for this purpose.