Easily finding and replacing every match in a nested list

Question

Take this object as an example:

expr <- substitute(mean(exp(sqrt(.)), .))

It is a nested list. I want to find every element that matches

Answer 1

Using a recursive function:

convert.call <- function(x, replacement) {
  if (is.call(x)) as.call(lapply(x, convert.call, replacement=replacement)) else
    if (identical(x, quote(.))) as.name(replacement) else
      x
}

convert.call(expr, "x")
# mean(exp(sqrt(x)), x)