Character code of unknown character-character, e.g. square or question mark romb

Question

What's the character code of the symbol that means an unknown character?

If a character isn't included in a font, it's often displayed as a square, indicating that the symbol doesn't exist in that font.

What's the character code of that square symbol? Not the code of the symbol that doesn't exist, but the actual square symbol?

I'd like the character code in UTF-8.

Answer 1

U+FFFD REPLACEMENT CHARACTER = 0xEF 0xBF 0xBD

Answer 2

Unicode has two symbols for unknown characters:

• □ (WHITE SQUARE, U+25A1) - Replaces a missing or unsupported Unicode character.
• � (REPLACEMENT CHARACTER, U+FFFD) - Replaces an invalid or unrecognizable character. Indicates a Unicode error.

Sources

• Quora - What symbol is the square box shown for non-representable Unicode characters?
• FileFormat.Info - Unicode Character 'WHITE SQUARE' (U+25A1)
• Wikipedia - Specials (Unicode block)