5.13 Character Translations

A translation is a mapping of an input character to an output glyph. The mapping occurs at output time, i.e., the input character gets assigned the metric information of the mapped output character right before tokens are converted to nodes (see GNU troff Internals, for more on this process).

Request: .tr abcd…

Request: .trin abcd…

Translate character a to glyph b, character c to glyph d, and so on. If there is an odd number of characters in the argument, the last one is translated to a fixed-width space (the same one obtained by the \SPC escape sequence).

The trin request works as does tr, except that asciify (see Diversions) ignores the translation when a diversion is interpolated.

Some notes:

• Special characters (\(xx, \[xxx], \C'xxx', \', \`, \-, \_), glyphs defined with the char request, and numbered glyphs (\N'xxx') can be translated also.
• The \e escape can be translated also.
• Characters can be mapped onto the \% and \~ escape sequences (but \% and \~ can’t be mapped onto another glyph).
• The following characters can’t be translated: space (with one exception, see below), backspace, newline, leader (and \a), tab (and \t).
• Translations are not considered for finding the soft hyphen character set with the shc request.
• The pair ‘c\&’ (an arbitrary character c followed by the dummy character) maps this character to “nothing”.

  .tr a\&
  foo bar
      ⇒ foo br
  

  Even the space character can be mapped to the dummy character.

  .tr aa \&
  foo bar
      ⇒ foobar
  

  As shown in the example, the space character can’t be the first character/glyph pair as an argument of tr. Additionally, it is not possible to map the space character to any other glyph; requests like ‘.tr aa x’ undo ‘.tr aa \&’ instead.

  If adjustment is enabled, it occurs in spite of the ‘empty’ space character; but no minimum distance—no minimum inter-word space—separates words).

• After an output glyph has been constructed (this happens at the moment immediately before the glyph is appended to an output glyph list, either by direct output, in a macro, diversion, or string), it is no longer affected by tr.
• Translating character to glyphs where one of them or both are undefined is possible also; tr does not check whether the elements of its argument exist.

  See GNU troff Internals.

• Without an argument, the tr request is ignored.

Request: .trnt abcd…

trnt is the same as the tr request except that the translations do not apply to text that is transparently throughput into a diversion with \!. See Diversions.

For example,

.tr ab
.di x
\!.tm a
.di
.x

prints ‘b’ to the standard error stream; if trnt is used instead of tr it prints ‘a’.