Escaping Backticks in String Interpolation in gsubfn

[brodieG]

a <- 10
b <- 5

fn$c("Hello $a + $b should equal `a + b`, but `don't evalute me`, $b")

I would want to be able to escape the second pair of backticks, or dollar signs e.g.:

fn$c("Hello $a + $b should equal `a + b`, but ``don't evalute me``, $$b")
## "Hello 10 + 5 should equal 15, but `don't evalute me`"

I think this can be achieved by changing pattern in:

if (missing(pattern)) pattern <- "[$]([[:alpha:]][[:alnum:].]*)|`([^`]+)`"

To something like (with perl=T):

"(?<!\\$)[$]([[:alpha:]][[:alnum:].]*)|`(?<!`)`([^`]+)`(?!`)`"

This type of escaping is probably simpler than anything involving backslashes given those have special meaning within R and with regexes as well. The only unfortunate thing is this is different to how the formula escaping works, where ~~ is the function.

Additionally, one complication is that we need to make sure that things like:

``don't evaluate me``

are not only not evaluated, but also converted to

`don't evaluate me`

[brodieG]

Also need to consider potential conflicts with knitr and the triple backtick (as per @ggrothendieck)