php - Regex\R 在字符类中不起作用

Question

在 PHP 中，应该匹配任何换行序列的转义字符 \R 在字符类中不起作用。

我最近在 stackoverflow 上的另一个答案中了解了这个特殊字符，老实说，我无法在网上找到太多来记录它的存在 - 除了在用户评论中之外，在 php.net 上没有任何地方提到它。

问题:

• 为什么 \R 不能在字符类中工作？
• 记录在哪里？

示例 1: ( https://regex101.com/r/vA8xV3/3 )

$a = "line1
      line2"

echo preg_replace('/\R/',' ',$a);

返回(找到匹配项，替换为单个空格):

line1 line2

示例 2: ( https://regex101.com/r/vA8xV3/2 )

$a = "line1
      line2"

echo preg_replace('/[\R]/',' ',$a);

返回(不匹配):

line1
line2

Answer 1

来自PCRE manual :

Escape sequences in character classes

All the sequences that define a single character value can be used both inside and outside character classes. In addition, inside a characterclass, \b is interpreted as the backspace character (hex 08).

\N is not allowed in a character class. \B, \R, and \X are not special inside a character class. Like other unrecognized escape sequences,they are treated as the literal characters "B", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is set. Outside acharacter class, these sequences have different meanings.

(强调我添加的相关位)