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A general description is here.
R now builds by default against a PCRE2 library for Perl-like regular expressions if available at build time on non-Windows platforms (otherwise PCRE1 is used as in older versions of R). On Windows, R always builds against PCRE2 by default.
PCRE2 is stricter in some cases and reports errors for some regular expressions that were accepted by PCRE1. A hyphen now has to be escaped in a character class to be interpreted as a literal (unless first or last in the class definition). ‘\R’, ‘\B’ and ‘\X’ are no longer allowed in character classes (PCRE1 treated these as literals).
Option ‘PCRE_study’ is no longer used, and reported as ‘FALSE’.
The version of PCRE used can be obtained via ‘extSoftVersion()’, PCRE1 has versions < 10, PCRE2 versions >= 10.