regex - Removing multiple commas and trailing commas using gsub

Question

This question is very similar to Removing multiple spaces and trailing spaces using gsub, except that I'd like to apply it to commas instead of spaces.

For example, I'd like a function TrimCommas to turn x into y:

x <- c("a,b,c", ",a,b,,c", ",,,a,,,b,c,,,")
# y <- TrimCommas(x) # presumably
y <- c("a,b,c", "a,b,c", "a,b,c")

The solution for spaces was gsub("^ *|(?<= ) | *$", "", x, perl=T), so I'm hoping comparing the solution for this will help explain some regex fundamentals as well.

Answer 1

Isn't the solution pretty similar?

x <- c("a,b,c", ",a,b,,c", ",,,a,,,b,c,,,")
gsub("^,*|(?<=,),|,*$", "", x, perl=T)
# [1] "a,b,c" "a,b,c" "a,b,c"

There are three parts to the regex ^,*|(?<=,),|,*$:

• ^,* -- this matches 0 or more commas at the beginning of the string
• (?<=,), -- this is a positive lookbehind to see if there a comma behind a comma, so it matches , in ,,
• ,*$ -- this matches 0 or more commas at the end of the string

As you can see all of the above are substituted with nothing.

You can make this generic to any character (" ", ",", etc.) with this function:

TrimMult <- function(x, char=" ") {
  return(gsub(paste0("^", char, "*|(?<=", char, ")", char, "|", char, "*$"),
              "", x, perl=T))
}