This compilation option was invented in order to turn on additional functionality of PCRE that is incompatible with Perl, but it is currently of very little use.

When set, any backslash in a pattern that is followed by a letter that has no special meaning causes an error, thus reserving these combinations for future expansion. By default, as in Perl, a backslash followed by a letter with no special meaning is treated as a literal.

There are at present no other features controlled by this option. It can also be set by a (?X) option setting within a pattern.