How to replace character at even position by ( and odd position by ) - regex

Question

Is it possible to write down a regular expression such that the first $ sign will be replaced by a (, the second with a ), the third with a (, etc ?
For instance, the string
This is an $example$ of what I want, $ 1+1=2 $ and $ 2+2=4$.
should become
This is an (example) of what I want, ( 1+1=2 ) and ( 2+2=4).

Answer 1

Sort of an indirect solution, but in some languages, you can use a callback function for the replacement. You can then cycle through the options in that function. This would also work with more than two options. For example, in Python:
>>> text = "This is an $example$ of what I want, $ 1+1=2 $ and $ 2+2=4$."
>>> options = itertools.cycle(["(", ")"])
>>> re.sub(r"\$", lambda m: next(options), text)
'This is an (example) of what I want, ( 1+1=2 ) and ( 2+2=4).'
Or, if those always appear in pairs, as it seems to be the case in your example, you could match both $ and everything in between, and then replace the $ and reuse the stuff in between using a group reference \1; but again, not all languages support those:
>>> re.sub(r"\$(.*?)\$", r"(\1)", text)
'This is an (example) of what I want, ( 1+1=2 ) and ( 2+2=4).'

Answer 2

According to an answer already posted here https://stackoverflow.com/a/13947249/6332575 in Ruby you can use
yourstring.gsub("$").with_index(1){|_, i| i.odd? ? "(" : ")"}

Answer 3

In JavaScript:
function replace$(str) {
let first = false;
return str.replace(/\$/, _ => (first = !first) ? '(' : ')');
}

Answer 4

In R, you can use str_replace, which only replaces the first match, and a while loop to deal with pairs of matches at a time.
# For str_*
library(stringr)
# For the pipes
library(magrittr)
str <- "asdfasdf $asdfa$ asdfasdf $asdf$ adsfasdf$asdf$"
while(any(str_detect(str, "\\$"))) {
str <- str %>%
str_replace("\\$", "(") %>%
str_replace("\\$", ")")
}
It's not the most efficient solution, probably, but it will go through and replace $ with ( and ) through the whole string.