What does a valid UTF-8 byte pattern look like?

In one line

The rule

In UTF-8, each character is encoded using 1 to 4 bytes. The first byte tells how many bytes are used, and each extra byte starts with '10' in binary.

Here are the patterns:

• 1-byte: 0xxxxxxx (for ASCII, 0–127)
• 2-byte: 110xxxxx 10xxxxxx
• 3-byte: 1110xxxx 10xxxxxx 10xxxxxx
• 4-byte: 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx

x means any bit (0 or 1).

Let's understand it simply

Imagine you are sending a message in a secret code. Each letter can be short (1 number) or long (2–4 numbers). To avoid confusion, you start each long letter with a special signal. In UTF-8, this signal is the pattern of bits at the start of each byte. For example, if the byte starts with '0', it is a simple letter (like English letters). If it starts with '110', it means, “This letter needs two bytes!”

How it works

1. Look at the first byte.
2. If it starts with '0', it's a 1-byte character (ASCII).
3. If it starts with '110', it's the start of a 2-byte character. The next byte must start with '10'.
4. If it starts with '1110', it's the start of a 3-byte character. The next two bytes must start with '10'.
5. If it starts with '11110', it's the start of a 4-byte character. The next three bytes must start with '10'.
6. Any byte after the first in a multi-byte character must start with '10'.

The story behind it

UTF-8 was invented by Ken Thompson and Rob Pike in 1992. They wanted a way to use all world languages on computers, but still keep old English text working. The goal was to make it easy for computers to spot mistakes and to keep files small if they only use English letters.

Where you see it in real life

• Web pages: Most websites use UTF-8 so you can see Hindi, Tamil, Chinese, and emoji together.
• WhatsApp messages: Sending a smiley or a Hindi word uses multi-byte UTF-8.
• Programming: When you save a Python or Java file, the text is often in UTF-8.
• File names: You can name a file 'नमस्ते.txt' because UTF-8 supports it.
• Databases: MySQL and others store text in UTF-8 to support all languages.

Why it matters

• Universal: Lets you write in any language, even mix them.
• Backward compatible: Old English text (ASCII) still works, as 1-byte UTF-8.
• Error detection: If a byte does not fit the pattern, you know something is wrong.
• Efficient: English text stays small, but you can add any symbol.

Common mistakes

• Thinking all characters are 1 byte: Many languages and emoji use 2–4 bytes.
• Not checking the '10' pattern: Continuation bytes must start with '10'.
• Mixing up UTF-8 with UTF-16: UTF-16 uses 2 or 4 bytes, not the same patterns.
• Assuming any byte value is valid: Only certain patterns are allowed in UTF-8.

Remember it

If a byte starts with '0', it's a single character. If it starts with '1', count the number of starting '1's to know how many bytes the character uses.