I'm trying to replace text a specific column of the line, hence the natural approach I saw is:

gsub("^(.{N})FOO", "\\1BAR", x)

i.e., capture whatever the first N characters are and, if they are followed by FOO, replace FOO with BAR (retaining whatever the first N characters were).

Here's my attempt in rex():

N = 12L
pattern <- rex(
  start,
  capture(n_times(anything, N)),
  "function"
)

But the resulting pattern generates infinite recursion (it seems):

grepl(ptn, "  lapply(y, function(yi) {", perl = TRUE)
# Warning message:
# In grepl(ptn, "  lapply(y, function(yi) {", perl = TRUE) :
#   back-tracking limit reached in PCRE for element 1

Actually, I am suspecting this might be an R / PCRE regex bug given how hard it was to come up with a more minimal reprex...