escaping - GNU m4 : escaping backticks (`)

Question

一个简单的GNU m4问题，但是我找不到正确的答案。我想在代码部分的开始/结束处打印一个markdown header :

```
echo Hello
```

如何创建包含3个反引号的GNU M4宏？就像是

define(`md_code',````')
md_code
echo Hello
md_code

Answer 1

在M4邮件列表上得到了Eric Blake的答复:http://lists.gnu.org/archive/html/m4-discuss/2015-04/msg00004.html

changequote is your friend. This will do it:

define(`md_code', changequote([, 
])[changequote([,])```changequote(`,')]changequote(`,'))

I have to change quotes twice: once around the macro definition, since the definition itself intends to use (backticks) in an unbalanced manner; and again in the macro expansion, since the expansion will output backticks in an unbalanced manner; for each changed quote, the original quotes must be restored. This assumes that the default quoting stays at (backticks) ' throughout the m4 run.

Although in your case, I'd recommend using changequote up front to something else, and globally write your input under those quoting rules instead of the default (backticks) ' quoting rules. Remember that autoconf intentionally went with [ ] as the quoting characters, because they were much likely to be balanced in output, as opposed to (backticks) and ' not occurring in balanced pairs in shell scripts. In fact, choosing 2- or 3-byte quoting strings is even less ambiguous, although it then requires more typing.