Regex group capture in R with multiple capture-groups

dd_69749 0
Viewed 61116

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?

capture-group capture r regex
9 Answers

str_match(), from the stringr package, will do this. It returns a character matrix with one column for each group in the match (and one for the whole match):

> s = c("(sometext :: 0.1231313213)", "(moretext :: 0.111222)")
> str_match(s, "\\((.*?) :: (0\\.[0-9]+)\\)")
     [,1]                         [,2]       [,3]          
[1,] "(sometext :: 0.1231313213)" "sometext" "0.1231313213"
[2,] "(moretext :: 0.111222)"     "moretext" "0.111222"
user_806260

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.

David_Lawrence_Miller_48920

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"))
dd_697490

This can be done using the package unglue, taking the example from the selected answer:

# install.packages("unglue")
library(unglue)

s <- c("(sometext :: 0.1231313213)", "(moretext :: 0.111222)")
unglue_data(s, "({x} :: {y})")
#>          x            y
#> 1 sometext 0.1231313213
#> 2 moretext     0.111222

Or starting from a data frame

df <- data.frame(col = s)
unglue_unnest(df, col, "({x} :: {y})",remove = FALSE)
#>                          col        x            y
#> 1 (sometext :: 0.1231313213) sometext 0.1231313213
#> 2     (moretext :: 0.111222) moretext     0.111222

you can get the raw regex from the unglue pattern, optionally with named capture :

unglue_regex("({x} :: {y})")
#>             ({x} :: {y}) 
#> "^\\((.*?) :: (.*?)\\)$"

unglue_regex("({x} :: {y})",named_capture = TRUE)
#>                     ({x} :: {y}) 
#> "^\\((?<x>.*?) :: (?<y>.*?)\\)$"

More info : https://github.com/moodymudskipper/unglue/blob/master/README.md

user_22704750
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