When writing an Autoconf macro you may occasionally need to generate
special characters that are difficult to express with the standard
Autoconf quoting rules. For example, you may need to output the regular
expression ‘[^’, which matches any character other than ‘[’.
This expression contains unbalanced brackets so it cannot be put easily
into an M4 macro.
You can work around this problem by using one of the following
- ‘@&[email protected]’
- Expands to nothing.
Quadrigraphs are replaced at a late stage of the translation process,
after m4 is run, so they do not get in the way of M4 quoting.
For example, the string ‘^@<:@’, independently of its quotation,
appears as ‘^[’ in the output.
The empty quadrigraph can be used:
- to mark trailing spaces explicitly
Trailing spaces are smashed by autom4te. This is a feature.
- to produce other quadrigraphs
For instance ‘@<@&[email protected]:@’ produces ‘@<:@’.
- to escape occurrences of forbidden patterns
For instance you might want to mention
AC_FOO in a comment, while
still being sure that autom4te still catches unexpanded
‘AC_*’. Then write ‘[email protected]&[email protected]_FOO’.
The name ‘@&[email protected]’ was suggested by Paul Eggert:
I should give some credit to the ‘@&[email protected]’ pun. The ‘&’ is my
own invention, but the ‘t’ came from the source code of the
algol68c compiler, written by Steve Bourne (of Bourne shell fame),
and which used ‘mt’ to denote the empty string. In C, it would
have looked like something like:
char const mt = "";
but of course the source code was written in Algol 68.
I don't know where he got ‘mt’ from: it could have been his own
invention, and I suppose it could have been a common pun around the
Cambridge University computer lab at the time.