Regex group capture in R

Question

In R, is it possible to extract group capture from a regular expression match? As far as I can tell, none of grep, grepl, regexpr, gregexpr, sub, or gsub return the group captures.

I need to extract key-value pairs from strings that are encoded thus:

\((.*?) :: (0\.[0-9]+)\)

I can always just do multiple full-match greps, or do some outside (non-R) processing, but I was hoping I can do it all within R. Is there's a function or a package that provides such a function to do this?

Answer 1

gsub does this, from your example:

gsub("\\((.*?) :: (0\\.[0-9]+)\\)","\\1 \\2", "(sometext :: 0.1231313213)")
[1] "sometext 0.1231313213"

you need to double escape the \s in the quotes then they work for the regex.

Hope this helps.

Answer 2

This is how I ended up working around this problem. I used two separate regexes to match the first and second capture groups and run two gregexpr calls, then pull out the matched substrings:

regex.string <- "(?<=\\().*?(?= :: )"
regex.number <- "(?<= :: )\\d\\.\\d+"

match.string <- gregexpr(regex.string, str, perl=T)[[1]]
match.number <- gregexpr(regex.number, str, perl=T)[[1]]

strings <- mapply(function (start, len) substr(str, start, start+len-1),
                  match.string,
                  attr(match.string, "match.length"))
numbers <- mapply(function (start, len) as.numeric(substr(str, start, start+len-1)),
                  match.number,
                  attr(match.number, "match.length"))