Like any other language, `gtroff' has rules for properly formed
"identifiers". In `gtroff', an identifier can be made up of almost any
printable character, with the exception of the following characters:
* Whitespace characters (spaces, tabs, and newlines).
* Backspace (ASCII `0x08' or EBCDIC `0x16') and character code
* The following input characters are invalid and are ignored if
`groff' runs on a machine based on ASCII, causing a warning
message of type `input' (see Note: Debugging, for more details):
`0x00', `0x0B', `0x0D'-`0x1F', `0x80'-`0x9F'.
And here are the invalid input characters if `groff' runs on an
EBCDIC host: `0x00', `0x08', `0x09', `0x0B', `0x0D'-`0x14',
Currently, some of these reserved codepoints are used internally,
thus making it non-trivial to extend `gtroff' to cover Unicode or
other character sets and encodings which use characters of these
Note that invalid characters are removed before parsing; an
identifier `foo', followed by an invalid character, followed by
`bar' is treated as `foobar'.
For example, any of the following is valid.
Note that identifiers longer than two characters with a closing bracket
(`]') in its name can't be accessed with escape sequences which expect
an identifier as a parameter. For example, `\[foo]]' accesses the
glyph `foo', followed by `]', whereas `\C'foo]'' really asks for glyph
To avoid problems with the `refer' preprocessor, macro names should
not start with `[' or `]'. Due to backwards compatibility, everything
after `.[' and `.]' is handled as a special argument to `refer'. For
example, `.[foo' makes `refer' to start a reference, using `foo' as a
-- Escape: \A'ident'
Test whether an identifier IDENT is valid in `gtroff'. It expands
to the character 1 or 0 according to whether its argument (usually
delimited by quotes) is or is not acceptable as the name of a
string, macro, diversion, number register, environment, or font.
It returns 0 if no argument is given. This is useful for looking
up user input in some sort of associative table.
Note: Escapes, for details on parameter delimiting characters.
Identifiers in `gtroff' can be any length, but, in some contexts,
`gtroff' needs to be told where identifiers end and text begins (and in
different ways depending on their length):
* Single character.
* Two characters. Must be prefixed with `(' in some situations.
* Arbitrary length (`gtroff' only). Must be bracketed with `['
and `]' in some situations. Any length identifier can be put in
Unlike many other programming languages, undefined identifiers are
silently ignored or expanded to nothing. When `gtroff' finds an
undefined identifier, it emits a warning, doing the following:
* If the identifier is a string, macro, or diversion, `gtroff'
defines it as empty.
* If the identifier is a number register, `gtroff' defines it with a
value of 0.
Note: Warnings., Note: Interpolating Registers, and Note:
Note that macros, strings, and diversions share the same name space.
. nop foo
As can be seen in the previous example, `gtroff' reuses the identifier
`xxx', changing it from a macro to a diversion. No warning is emitted!
The contents of the first macro definition is lost.
Note: Interpolating Registers, and Note: Strings.
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