Ersatzzeichen

What Is the Replacement Character?

The Replacement Character is U+FFFD (� — displayed as \ufffd or the familiar diamond with a question mark <> symbol). It is the designated substitute character that Unicode and encoding systems insert when they encounter a byte sequence that cannot be decoded, is invalid for the declared encoding, or represents a code point that has no valid mapping.

The character was chosen because its code point 0xFFFD is in the Specials block and has no other assigned meaning — it exists solely as an error sentinel.

When It Appears

1. Invalid UTF-8 sequences: A byte in a range reserved for multi-byte sequences but not followed by the correct continuation bytes.
2. Truncated sequences: A multi-byte UTF-8 sequence cut off at the end of a buffer.
3. Surrogates in UTF-8: Lone or paired surrogates (U+D800–U+DFFF) encoded in UTF-8, which is technically invalid.
4. Out-of-range code points: Code points above U+10FFFF are not Unicode.
5. Unmappable characters: When converting between encodings and a character has no equivalent.

# UTF-8 decoding with error=replace
b"\xff\xfe".decode("utf-8", errors="replace")  # "\ufffd\ufffd"
b"\xe4\xb8".decode("utf-8", errors="replace")  # "\ufffd" (truncated CJK)
b"\xed\xa0\x80".decode("utf-8", errors="replace")  # "\ufffd\ufffd\ufffd" (surrogate in UTF-8)

# str: U+FFFD as a Python character
REPLACEMENT = "\uFFFD"
REPLACEMENT == "�"  # Same character
ord(REPLACEMENT)           # 65533

# Check for replacement characters in decoded text
def has_decoding_errors(text: str) -> bool:
    return "\uFFFD" in text

JavaScript

const decoder = new TextDecoder("utf-8");  // fatal=false by default → uses U+FFFD
const badBytes = new Uint8Array([0xFF, 0xFE]);
decoder.decode(badBytes);  // "\ufffd\ufffd"

// Fatal mode: throws instead of replacing
const strictDecoder = new TextDecoder("utf-8", { fatal: true });
try {
  strictDecoder.decode(badBytes);  // TypeError: The encoded data was not valid
} catch (e) {
  console.log("Invalid bytes");
}

// Check for replacement character
"\ufffd".codePointAt(0);     // 65533
"\ufffd" === "\u{FFFD}";     // true

HTML Rendering

In HTML, � and � render as the replacement character glyph. Some fonts render it as a black diamond <>, others as a question mark in a box, or just ?.

<p>Invalid sequence: &#xFFFD;</p>
<!-- Browsers display the replacement character glyph -->

Database and File Handling

Replacement characters in stored data indicate an encoding problem that already occurred. They cannot be "fixed" because the original bytes are gone — the information was lost at decode time:

# Once decoded with errors="replace", the original byte is unrecoverable
bad = b"\x80"
replaced = bad.decode("utf-8", errors="replace")  # "\ufffd"
# You cannot go back to b"\x80" from "\ufffd" alone

# Solution: store original bytes if you need to recover them
import base64
preserved = base64.b64encode(bad).decode()  # "gA==" — recoverable

Normalization and Filtering

In data pipelines, you should decide deliberately whether to keep or remove replacement characters:

# Filter replacement characters (data was corrupted — remove noise)
def clean_text(text: str) -> str:
    return text.replace("\uFFFD", "")

# Count corruption severity
def corruption_ratio(text: str) -> float:
    if not text:
        return 0.0
    return text.count("\uFFFD") / len(text)

Quick Facts