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A general description is here.
The parser now supports C-preprocessor-like #line directives, so error messages and source references may refer to the original file rather than an intermediate one.
New functions findLineNum() and setBreakpoint() work with the source references to find the location of source lines and set breakpoints (using trace()) at those lines.
There is a different regular expression engine for basic and extended regexps, based on the TRE library. This is often faster, especially in a MBCS locale, and it allows us to implement the same set of options as for fixed = TRUE and perl = TRUE, including 'useBytes' and full support for UTF-8 encoded strings in all locales.-encoded strings in all locales.
One known difference is that it is less tolerant of invalid inputs in MBCS locales, and conforms more strictly to the POSIX standard in its interpretation of incorrect regexps such as "^*".
[Currently experimental, can be deselected by the configure option --without-TRE.]