A named logical vector, currently with elements
UTF-8Support for UTF-8 inputs.  Required.
  Unicode propertiesSupport for \p{xx} and \P{xx}
    in regular expressions.  Desirable and used by some CRAN packages.
  JITSupport for just-in-time compilation.  Desirable for speed
    (but only available as a compile-time option on certain
    architectures).
  stackDoes match recursion use a stack (TRUE, the PCRE
    default) or a heap?  See the discussion at
    http://www.pcre.org/original/doc/html/pcrestack.html.  (Added
    in R 3.4.0.)