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.