For non-printable and certain special (well-known,
see also the R man page Quotes)
ASCII characters, the following
(also recognized in R) convention is used.
We get \a
, \b
, \t
, \n
, \v
,
\f
, \r
, \"
, \'
, \\
or either \uXXXX
(4 hex digits) or \UXXXXXXXX
(8 hex digits)
otherwise.
As usual in stringi, any input string is converted to Unicode
before executing the escape process.