8.1.4 Quadrigraphs

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 quadrigraphs:

`@<:@'

`['

`@:>@'

`]'

`@S|@'

`$'

`@%:@'

`#'

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 `[^@<:@]', if properly quoted, will appear as `[^[]' in the configure script.