regex - 是否有正则表达式可以检测有效的正则表达式？

Question

是否可以用另一个正则表达式来检测有效的正则表达式？如果是这样，请在下面给出示例代码。

Answer 1

/
^                                             # start of string
(                                             # first group start
  (?:
    (?:[^?+*{}()[\]\\|]+                      # literals and ^, $
     | \\.                                    # escaped characters
     | \[ (?: \^?\\. | \^[^\\] | [^\\^] )     # character classes
          (?: [^\]\\]+ | \\. )* \]
     | \( (?:\?[:=!]|\?<[=!]|\?>)? (?1)?? \)  # parenthesis, with recursive content
     | \(\? (?:R|[+-]?\d+) \)                 # recursive matching
     )
    (?: (?:[?+*]|\{\d+(?:,\d*)?\}) [?+]? )?   # quantifiers
  | \|                                        # alternative
  )*                                          # repeat content
)                                             # end first group
$                                             # end of string
/

这是一个递归正则表达式，许多正则表达式引擎不支持。基于 PCRE 的应该支持它。

没有空格和注释:

/^((?:(?:[^?+*{}()[\]\\|]+|\\.|\[(?:\^?\\.|\^[^\\]|[^\\^])(?:[^\]\\]+|\\.)*\]|\((?:\?[:=!]|\?<[=!]|\?>)?(?1)??\)|\(\?(?:R|[+-]?\d+)\))(?:(?:[?+*]|\{\d+(?:,\d*)?\})[?+]?)?|\|)*)$/

<小时/>

.NET 不直接支持递归。 ((?1) 和 (?R) 构造。)递归必须转换为计算平衡组:

^                                         # start of string
(?:
  (?: [^?+*{}()[\]\\|]+                   # literals and ^, $
   | \\.                                  # escaped characters
   | \[ (?: \^?\\. | \^[^\\] | [^\\^] )   # character classes
        (?: [^\]\\]+ | \\. )* \]
   | \( (?:\?[:=!]
         | \?<[=!]
         | \?>
         | \?<[^\W\d]\w*>
         | \?'[^\W\d]\w*'
         )?                               # opening of group
     (?<N>)                               #   increment counter
   | \)                                   # closing of group
     (?<-N>)                              #   decrement counter
   )
  (?: (?:[?+*]|\{\d+(?:,\d*)?\}) [?+]? )? # quantifiers
| \|                                      # alternative
)*                                        # repeat content
$                                         # end of string
(?(N)(?!))                                # fail if counter is non-zero.

压缩:

^(?:(?:[^?+*{}()[\]\\|]+|\\.|\[(?:\^?\\.|\^[^\\]|[^\\^])(?:[^\]\\]+|\\.)*\]|\((?:\?[:=!]|\?<[=!]|\?>|\?<[^\W\d]\w*>|\?'[^\W\d]\w*')?(?<N>)|\)(?<-N>))(?:(?:[?+*]|\{\d+(?:,\d*)?\})[?+]?)?|\|)*$(?(N)(?!))

来自评论:

Will this validate substitutions and translations?

它将仅验证替换和翻译的正则表达式部分。 s/<this part>/.../

It is not theoretically possible to match all valid regex grammars with a regex.

如果正则表达式引擎支持递归，例如PCRE，这是可能的，但那不能再称为正则表达式了。

Indeed, a "recursive regular expression" is not a regular expression. But this an often-accepted extension to regex engines... Ironically, this extended regex doesn't match extended regexes.

“从理论上讲，理论和实践是相同的。但在实践中，它们却不相同。”几乎所有了解正则表达式的人都知道，正则表达式不支持递归。但 PCRE 和大多数其他实现支持的不仅仅是基本正则表达式。

using this with shell script in the grep command , it shows me some error.. grep: Invalid content of {} . I am making a script that could grep a code base to find all the files that contain regular expressions

此模式利用了称为递归正则表达式的扩展。 POSIX 风格的正则表达式不支持这一点。您可以尝试使用 -P 开关来启用 PCRE 正则表达式风格。

Regex itself "is not a regular language and hence cannot be parsed by regular expression..."

这对于经典正则表达式来说是正确的。一些现代实现允许递归，这使其成为上下文无关语言，尽管对于此任务来说有点冗长。

I see where you're matching []()/\. and other special regex characters. Where are you allowing non-special characters? It seems like this will match ^(?:[\.]+)$, but not ^abcdefg$. That's a valid regex.

[^?+*{}()[\]\\|]将匹配任何单个字符，而不是任何其他结构的一部分。这包括文字( a - z )和某些特殊字符( ^ 、 $ 、 . )。