Modify a substring matched by regex and place it back [duplicate]

Question

I have an arbitrary string, say "1a2 2a1 3a2 10a5" I want to do an arbitrary mathematical operation, say doubling, to some of the numbers, say anything followed by an "a".

I can extract the numbers I want with relative ease

string = "1a2 2a1 3a2 10a5"
numbers = stringr::str_extract_all(string,'[0-9]+(?=a)')[[1]]

and obviously, doubling them is trivial

numbers = 2*(as.integer(numbers))

But I am having problems with placing the new results in their old positions. To get the output "2a2 4a1 6a2 20a5". I feel like there should be a single function that accomplishes this but all I can think of is recording the original indexes of the matches using gregexpr and manually placing the new results in the coordinates.

Answer 1

We can use str_replace_all from stringr to capture numbers followed by "a" and then multiply them by 2 in their callback function.

stringr::str_replace_all(string, "\\d+(?=a)", function(m) as.integer(m) * 2)
#[1] "2a2 4a1 6a2 20a5"