Rearrange a string using regex

June 5, 2023

I have an expression as a string, like the following:

orig <- "mean(Sepal.Length, na.rm = TRUE)"
orig
#> [1] "mean(Sepal.Length, na.rm = TRUE)"

I would like to rearrange this string so that I get the following output:

"Sepal.Length$mean(na.rm = TRUE)"
#> [1] "Sepal.Length$mean(na.rm = TRUE)"

I know that I can use capture groups like this:

gsub("(Sepal.Length)", "\\1\\$", orig)
#> [1] "mean(Sepal.Length$, na.rm = TRUE)"

but this doesn’t work to move text in the string:

gsub("(Sepal.Length)(.*)", "\\1\\$\\2", orig)
#> [1] "mean(Sepal.Length$, na.rm = TRUE)"

This question was helpful but the solution there is hardcoded, whereas here I don’t know the expression I will have at all, just that it will contain Sepal.Length. The expression above could be "sum(Sepal.Length)" for example.

I’m looking for a solution in base R only.

>Solution :

You can use the following pattern:

gsub("(.+)\\(Sepal\\.Length, *(.+)\\)", "Sepal.Length$\\1(\\2)", orig)

(.+) matches anything before the first parenthesis \\(;
Then, we always have "Sepal.Length". Note that . is a special character, so to use a literal dot you need \\.;
We then have a ,, maybe a space also * (* means "0 or more times");
After that we have the other arguments (.+), followed by the closing parenthesis \\).