5.1.9 Input Format

Organize input to GNU troff into lines separated by the Unix newline character (U+000A), using the character encoding it recognizes: ISO Latin-1 (8859-1). A document encoded in ISO 646:1991 IRV (US-ASCII), or, equivalently, uses only code points from the “C0 Controls” and “Basic Latin” parts of the Unicode character set is also a valid ISO Latin-1 document; the standards are interchangeable in their first 128 code points.³⁴

Some control characters (from the sets “C0 Controls” and “C1 Controls” as Unicode describes them) are invalid as input characters. GNU troff discards them upon reading.³⁵ It processes a character sequence “foo”, followed by an invalid character and then “bar”, as “foobar”.

Invalid input characters comprise 0x00, 0x0B, 0x0D–0x1F, and 0x80–0x9F.³⁶ GNU troff uses some of these code points for internal purposes, making non-trivial the extension of the program to accept UTF-8 or other encodings that use characters from these ranges.