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A general description is here.
R now builds by default against 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).
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.
Either PCRE >= 8.32 (Nov 2012) or PCRE2 is required: the deprecated provision for 8.20-8.31 has been removed.
The minimum required version of ‘libcurl’ is now 7.28.0 (Oct 2012).
PCRE2 is used if available: this is strongly recommended as PCRE1 is no longer developed (it only receives bug fixes and has outdated Unicode tables). ‘configure’ will warn if PCRE2 is not found and fall back to PCRE1. ‘configure’ options ‘--without-pcre1’ and ‘--without-pcre2’ can be used to exclude one or the other (but at least one of PCRE2 and PCRE1 is required).