The function returns the regular expression pattern that identifies
all r engines in Rmd chunks. Defaults to [Rr]. You probably only want to
change this if you create a knitr engine that processes R code but is not
the default engine r.
The pattern must be followed by a space (in the case the chunk is given
a name), a comma (if no name is given but further options are passed to the
engine) or a closing curly brace (in case no option and no name is given to
the chunk).
get_engine_pattern()