Regex group capture in R with multiple capture-groups
RegexRCaptureCapture GroupRegex Problem Overview
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?
Regex Solutions
Solution 1 - Regex
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"
Solution 2 - Regex
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.
Solution 3 - Regex
Try regmatches() and regexec():
regmatches("(sometext :: 0.1231313213)",regexec("\\((.*?) :: (0\\.[0-9]+)\\)","(sometext :: 0.1231313213)"))
[[1]]
[1] "(sometext :: 0.1231313213)" "sometext" "0.1231313213"
Solution 4 - Regex
gsub() can do this and return only the capture group:
However, in order for this to work, you must explicitly select elements outside your capture group as mentioned in the gsub() help.
> (...) elements of character vectors 'x' which are not substituted will be returned unchanged.
So if your text to be selected lies in the middle of some string, adding .* before and after the capture group should allow you to only return it.
gsub(".*\\((.*?) :: (0\\.[0-9]+)\\).*","\\1 \\2", "(sometext :: 0.1231313213)") [1] "sometext 0.1231313213"
Solution 5 - Regex
Solution with strcapture from the utils:
x <- c("key1 :: 0.01",
"key2 :: 0.02")
strcapture(pattern = "(.*) :: (0\\.[0-9]+)",
x = x,
proto = list(key = character(), value = double()))
#> key value
#> 1 key1 0.01
#> 2 key2 0.02
Solution 6 - Regex
I like perl compatible regular expressions. Probably someone else does too...
Here is a function that does perl compatible regular expressions and matches the functionality of functions in other languages that I am used to:
regexpr_perl <- function(expr, str) {
match <- regexpr(expr, str, perl=T)
matches <- character(0)
if (attr(match, 'match.length') >= 0) {
capture_start <- attr(match, 'capture.start')
capture_length <- attr(match, 'capture.length')
total_matches <- 1 + length(capture_start)
matches <- character(total_matches)
matches[1] <- substr(str, match, match + attr(match, 'match.length') - 1)
if (length(capture_start) > 1) {
for (i in 1:length(capture_start)) {
matches[i + 1] <- substr(str, capture_start[[i]], capture_start[[i]] + capture_length[[i]] - 1)
}
}
}
matches
}
Solution 7 - Regex
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"))
Solution 8 - Regex
As suggested in the stringr package, this can be achieved using either str_match() or str_extract().
Adapted from the manual:
library(stringr)
strings <- c(" 219 733 8965", "329-293-8753 ", "banana",
"239 923 8115 and 842 566 4692",
"Work: 579-499-7527", "$1000",
"Home: 543.355.3679")
phone <- "([2-9][0-9]{2})[- .]([0-9]{3})[- .]([0-9]{4})"
Extracting and combining our groups:
str_extract_all(strings, phone, simplify=T)
# [,1] [,2]
# [1,] "219 733 8965" ""
# [2,] "329-293-8753" ""
# [3,] "" ""
# [4,] "239 923 8115" "842 566 4692"
# [5,] "579-499-7527" ""
# [6,] "" ""
# [7,] "543.355.3679" ""
Indicating groups with an output matrix (we're interested in columns 2+):
str_match_all(strings, phone)
# [[1]]
# [,1] [,2] [,3] [,4]
# [1,] "219 733 8965" "219" "733" "8965"
#
# [[2]]
# [,1] [,2] [,3] [,4]
# [1,] "329-293-8753" "329" "293" "8753"
#
# [[3]]
# [,1] [,2] [,3] [,4]
#
# [[4]]
# [,1] [,2] [,3] [,4]
# [1,] "239 923 8115" "239" "923" "8115"
# [2,] "842 566 4692" "842" "566" "4692"
#
# [[5]]
# [,1] [,2] [,3] [,4]
# [1,] "579-499-7527" "579" "499" "7527"
#
# [[6]]
# [,1] [,2] [,3] [,4]
#
# [[7]]
# [,1] [,2] [,3] [,4]
# [1,] "543.355.3679" "543" "355" "3679"
Solution 9 - Regex
This can be done using the package unglue, taking the example from the selected answer:
# install.packages("unglue")
library(unglue)
s <- c("(sometext :: 0.1231313213)", "(moretext :: 0.111222)")
unglue_data(s, "({x} :: {y})")
#> x y
#> 1 sometext 0.1231313213
#> 2 moretext 0.111222
Or starting from a data frame
df <- data.frame(col = s)
unglue_unnest(df, col, "({x} :: {y})",remove = FALSE)
#> col x y
#> 1 (sometext :: 0.1231313213) sometext 0.1231313213
#> 2 (moretext :: 0.111222) moretext 0.111222
you can get the raw regex from the unglue pattern, optionally with named capture :
unglue_regex("({x} :: {y})")
#> ({x} :: {y})
#> "^\\((.*?) :: (.*?)\\)$"
unglue_regex("({x} :: {y})",named_capture = TRUE)
#> ({x} :: {y})
#> "^\\((?<x>.*?) :: (?<y>.*?)\\)$"
More info : https://github.com/moodymudskipper/unglue/blob/master/README.md