Adding letter to a string vector in R

Question

Disregarding the hashtaged lines in my input, is there a way to add:

• A* before "alphanumeric" elements that don't have a * prefix and appear after ~ and
• R* before "alphanumeric" elements that don't have a * prefix and appear after ~~?

Desired output is shown below.

input <- "y ~ x1 + x2
           ## Variances of x1 and x2 are 1
           x1 ~~ 1*x1
           x2 ~~ 1*x2
           ## x1 and x2 are correlated
           x1 ~~ x2"

output <- "y ~ A*x1 + A*x2
           ## Variances of x1 and x2 are 1
           x1 ~~ 1*x1
           x2 ~~ 1*x2
           ## x1 and x2 are correlated
           x1 ~~ R*x2"

Answer 1

You can use stringr::str_replace_all() and make use of the match groups. In the example below I am using three stages to achieve the result you specify.

library(stringr)

res <-
  str_replace_all(input,
                  " ~ ([a-z])",
                  " ~ A*\\1") |>
  str_replace_all("\\+ ([a-z])",
                  "+ A*\\1") |>
  str_replace_all("~~ ([a-z])",
                  "~~ R*\\1")

cat(res)
#> y ~ A*x1 + A*x2
#>          ## Variances of x1 and x2 are 1
#>          x1 ~~ 1*x1
#>          x2 ~~ 1*x2
#>          ## x1 and x2 are correlated
#>          x1 ~~ R*x2