The set_regexpr_style() allows user to specify which style to be used,
while the auto_set_regexpr_style() automatically set the style depdending on
the operating system (TRE for Windows and PCRE for other OSs (Linux and Mac)).
set_PCRE()
set_TRE()
use_perl()
set_regexpr_style(style)
auto_set_regexpr_style()logical value of whether use perl
set_PCRE() force to use PCRE style while set_TRE() force to use TRE.
Note that all these functions are not change the behavior of gsub() and regexpr().
The functions are just set a global option to store the user's choice of whether using perl = TRUE.
Users can access the option via use_perl() and pass the return value to gusb() or regexpr() to specify the style in use.