Regex \R doesn't work inside character class

Question

In PHP, the escape character \R which should match any newline sequence doesn't work inside a character class.

I recently learned about this special character on another answer here on stackoverflow and to be honest I haven't been able to find much online to document it's existence - nowhere on php.net is it mentioned except in a user comment.

Question(s):

• Why won't \R work inside a character class?
• Where is it documented?

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

$a = "line1
      line2"

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

Returns (finds match, replace with single space):

line1 line2

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

$a = "line1
      line2"

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

Returns (no match):

line1
line2

Answer 1

From the 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.

(emphasis on relevant bit added by me)

Answer 2

This is correct behavior. \R only works outside character class. (At least this is true in grep an many others)

For grep:

https://stat.ethz.ch/R-manual/R-devel/library/base/html/regex.html

PHP uses perl-like expressions, so see peardoc:

http://perldoc.perl.org/perlrebackslash.html#Misc

Since \R can match a sequence of more than one character, it cannot be put inside a bracketed character class; /[\R]/ is an error; use \v instead