What Ghostchars detects

Characters with no glyph and no width that are contraband anywhere in prose: zero width space, word joiner, soft hyphen, the byte order mark, the four invisible maths operators, deprecated format controls, interlinear annotation marks, musical-notation controls and Unicode’s permanently reserved noncharacters.

The five embedding and override controls. They reorder text that is not theirs, which is how a filename can be displayed as something it is not. There is no context in which Ghostchars keeps one.

Direction hints — the left-to-right and right-to-left marks, the Arabic letter mark, and the four isolates. Next to right-to-left text they resolve genuine ambiguity and are kept; in an English paragraph they are invisible padding and a published steganography carrier, and they go.

Invisible characters that bind to the character before or after them: zero width joiner and non-joiner, variation selectors, tag characters, and the script-specific fillers of Mongolian, Khmer and Hangul. Kept when their neighbours make them load-bearing, removed when they are free-floating.

The catch-all. Every remaining character whose Unicode general category is Cf is removed, so a format control added in a future Unicode version is caught without a code change. The orthographic exceptions — the Arabic number signs, the Syriac abbreviation mark, the Kaithi number sign — are matched earlier and left alone.

Detected by Unicode property rather than from a list, so there is no fixed set of codepoints to enumerate.

Codepoints Unicode will never assign, whose meaning is a private agreement between a font and an application. Icon fonts live here, which is why removing them is opt-in: in UI copy they are the content.

Spaces that are not U+0020: the no-break space, the narrow and medium ones, the ideographic space, the Ogham space mark and the eleven fixed-width typographic spaces. They are replaced with an ordinary space rather than deleted, because they are separating words.

Unicode’s line and paragraph separators and the EBCDIC-derived NEL. Folded to a plain line feed rather than deleted — deleting them would run two lines together.

The C0 and C1 control codes, excluding tab, line feed and carriage return. Their presence in text usually means a decoding accident or a deliberate probe.

Curly quotes, em and en dashes, the ellipsis, the minus sign and the primes. Nothing is hidden here — these are visible characters — but they are a different codepoint from their ASCII forms, and the straight-versus-curly choice is a published one-bit-per-apostrophe channel.

Cyrillic, Greek, Armenian, Cherokee, fullwidth and mathematical letters that are drawn identically to Latin ones. The mapping has 1,513 entries generated from Unicode’s own confusables data. Opt-in, because applying it destroys genuine non-Latin text.