/usr/share/doc/brltty/README-CTB is in brltty 4.3-1ubuntu5.
This file is owned by root:root, with mode 0o644.
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Files with names of the form *.ctb are contraction tables, and with names of
the form *.cti are contraction subtables. They are used by BRLTTY to translate
character sequences on the screen into their corresponding contracted braille
representations.
A contraction table consists of a sequence of entries, one per line, which
define how character sequences are to be represented in braille. UTF-8
character encoding must be used. White-space (blanks, tabs) at the beginning of
a line, as well as before and/or after any operand, is ignored. Lines
containing only white-space are ignored. If the first non-white-space character
of a line is "#" then that line is a comment and is ignored.
The format of a contraction table entry is:
directive operand ... [comment]
Each directive has a specific number of operands. Any text beyond the last
operand of a directive is interpreted as a comment. The order of the entries
within a contraction table is, in general, anything that is convenient for its
maintainer(s). An entry which defines an entity, e.g. class, must precede all
references to that entity.
Entries which match character sequences are automatically rearranged from
longest to shortest so that longer matches are always preferred. If more than
one entry matches the same character sequence then their original table
ordering is maintained. Thus, the same sequence may be translated differently
under different circumstances.
The Characters Operand
----------------------
The first operand of a character sequence matching directive is the character
sequence to be matched. Each character within the sequence may be:
* Any single character other than a backslash or a white-space character.
* A backslash-prefixed special character. These are:
- \b The backspace character.
- \f The formfeed character.
- \n The newline character.
- \o### The three-digit octal representation of a character.
- \r The carriage return character.
- \s The space character.
- \t The horizontal tab character.
- \u#### The four-digit hexadecimal representation of a character.
- \U######## The eight-digit hexadecimal representation of a character.
- \v The vertical tab character.
- \x## The two-digit hexadecimal representation of a character.
- \X## ... (the case of the X and of the digits isn't significant)
- \# A literal number sign.
- \<name> The Unicode name of a character (use _ for space).
- \\ A literal backslash.
The Representation Operand
--------------------------
The second operand of those character sequence matching directives which have
one is the braille representation of the sequence. Each braille cell is
specified as a sequence of one to eight dot numbers. A dot number is a digit
within the range 1-8 as defined by the standard braille dot numbering
convention (see README-DOTS). The special dot number 0, which may not be used
in conjunction with any other dot number, means no dots.
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