parsing - 使用 Pest.rs，我如何指定要锚定和整行的评论？

Question

Exim uses a really awkward comment syntax,

Blank lines in the file, and lines starting with a # character (ignoring leading white space) are treated as comments and are ignored. Note: A # character other than at the beginning of a line is not treated specially, and does not introduce a comment.

这意味着，

# This is is a comment
This has no comments # at all

有没有办法用 Pest.rs 反射(reflect)这一点？这个我试过了

COMMENT    = { "#" ~ (!NEWLINE ~ ANY)* ~ NEWLINE }
WHITESPACE = _{ " " }
main       = { SOI ~ ASCII_ALPHA* ~ EOI }

但是，这将匹配

MyText # Exim test this is not a comment 

如何将评论锚定到左侧？

Answer 1

这在默认 COMMENT 下是不可能的扩展，因为它已扩展为 all instances of rule-concatenation with ~ except for the atomics. .以下两行相同，

a = { b ~ c }
a = { b ~ WHITESPACE* ~ (COMMENT ~ WHITESPACE*)* ~ c }

这实质上意味着如果您要使用 ~和 COMMENT您必须将规则限制为 atomic rules with @ and $
而不是这个，对于基于行的语法，我最终改进了这个，而不是使用 COMMENT宏。而是定义我自己的宏， _COMMENT避免正常扩展为非原子标记，

WHITESPACE = _{ " " }
_COMMENT   = { "#" ~ (!NEWLINE ~ ANY)* ~ NEWLINE }
expr       = { ASCII_ALPHA+ }
stmt       = { expr ~ NEWLINE }
conf       = { SOI ~ (stmt | _COMMENT | NEWLINE)+ ~ EOI }

注意这里 stmt和 _COMMENT是 NEWLINE终止，并且 conf 包含 1 个或多个。