r - 为什么这个正则表达式模式匹配这个字符串？

Question

图案:

"d`?(?!([\\s]*<-))"

字符串:

"d` <-"

根据R，这是一场比赛:

> grepl("d`?(?!([\\s]*<-))", "d` <-", perl = TRUE)
[1] TRUE

这对我来说没有意义，因为 d匹配，(0 或 1)反引号匹配。但是 " <-"不应该是一场比赛吗？

Answer 1

原因是回溯，如 Wiktor Stribizew's comment 中所述。 .
将可选的反引号添加到前瞻中并将前瞻移动到 d 之后:

d(?!`?(\s*<-))`?

见 proof
说明

  [a-z]                    any character of: 'a' to 'z'
--------------------------------------------------------------------------------
  \d{2,4}                  digits (0-9) (between 2 and 4 times
                           (matching the most amount possible))
--------------------------------------------------------------------------------
  (?:                      group, but do not capture (optional
                           (matching the most amount possible)):
--------------------------------------------------------------------------------
    -                        '-'
--------------------------------------------------------------------------------
    [a-z]                    any character of: 'a' to 'z'
--------------------------------------------------------------------------------
    \d{2,4}                  digits (0-9) (between 2 and 4 times
                             (matching the most amount possible))
--------------------------------------------------------------------------------
  )?                       end of grouping
--------------------------------------------------------------------------------
  (?:                      group, but do not capture (optional
                           (matching the most amount possible)):
--------------------------------------------------------------------------------
    -                        '-'
--------------------------------------------------------------------------------
    [a-z]                    any character of: 'a' to 'z'
--------------------------------------------------------------------------------
  )?                       end of grouping