I have received a mail from the CRAN maintainers because I have a CRAN package that imports re2r:

re2r will unfortunately now be removed/archived by the CRAN maintainers due to CRAN check problems (see below) (some of which seem to be addressed by @Pilipino in #21).

re2r works in many cases much faster than the other available regex implementations in R, so this is very regrettable indeed. I do not myself have the necessary C/C++ etc. competency to fix the issues.

Copied from https://cran.r-project.org/web/checks/check_results_re2r.html on 4 March 2020:

Check Details
Version: 0.2.0
Check: for GNU extensions in Makefiles
Result: NOTE
GNU make is a SystemRequirements.
Flavors: r-devel-linux-x86_64-debian-clang, r-devel-linux-x86_64-debian-gcc, r-devel-linux-x86_64-fedora-clang, r-devel-linux-x86_64-fedora-gcc, r-devel-windows-ix86+x86_64, r-devel-windows-ix86+x86_64-gcc8, r-patched-linux-x86_64, r-patched-solaris-x86, r-release-linux-x86_64, r-release-windows-ix86+x86_64, r-release-osx-x86_64, r-oldrel-windows-ix86+x86_64, r-oldrel-osx-x86_64

Version: 0.2.0
Check: pragmas in C/C++ headers and code
Result: NOTE
Files which contain pragma(s) suppressing diagnostics:
'inst/include/re2/prog.h' 'inst/include/re2/re2.h'
'inst/include/re2/regexp.h' 'src/dfa.cc' 'src/onepass.cc'
Flavors: r-devel-linux-x86_64-debian-clang, r-devel-linux-x86_64-debian-gcc, r-devel-linux-x86_64-fedora-clang, r-devel-linux-x86_64-fedora-gcc, r-devel-windows-ix86+x86_64, r-devel-windows-ix86+x86_64-gcc8, r-patched-linux-x86_64, r-release-linux-x86_64, r-release-windows-ix86+x86_64, r-oldrel-windows-ix86+x86_64

Version: 0.2.0
Check: whether package can be installed
Result: WARN
Found the following significant warnings:
compile.cc:292:69: warning: ‘void* memset(void*, int, size_t)’ clearing an object of non-trivial type ‘class re2::Prog::Inst’; use assignment or value-initialization instead [-Wclass-memaccess]
regexp.cc:343:33: warning: ‘void* memmove(void*, const void*, size_t)’ writing to an object of type ‘class re2::Regexp’ with no trivial copy-assignment [-Wclass-memaccess]
regexp.cc:344:32: warning: ‘void* memmove(void*, const void*, size_t)’ writing to an object of type ‘class re2::Regexp’ with no trivial copy-assignment [-Wclass-memaccess]
Flavors: r-devel-linux-x86_64-debian-gcc, r-devel-linux-x86_64-fedora-gcc, r-devel-windows-ix86+x86_64-gcc8, r-patched-linux-x86_64, r-release-linux-x86_64

Version: 0.2.0
Check: installed package size
Result: NOTE
installed size is 11.7Mb
sub-directories of 1Mb or more:
doc 1.5Mb
libs 9.0Mb
Flavors: r-devel-linux-x86_64-fedora-clang, r-devel-windows-ix86+x86_64, r-devel-windows-ix86+x86_64-gcc8, r-release-windows-ix86+x86_64, r-release-osx-x86_64, r-oldrel-windows-ix86+x86_64, r-oldrel-osx-x86_64

Version: 0.2.0
Check: for unstated dependencies in ‘tests’
Result: WARN
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Warning in deparse(e[[2L]]) :
it is not known that wchar_t is Unicode on this platform
Flavor: r-patched-solaris-x86