Given a regular expression containing capture groups (parentheses) and a string, how can I obtain all the substrings matching the capture groups, i.e., the substrings usually referenced by "\1", "\2"?
Example: consider a regex capturing digits preceded by "xy":
s <- "xy1234wz98xy567"
r <- "xy(\\d+)"
Desired result:
[1] "1234" "567"
First attempt: gregexpr:
regmatches(s,gregexpr(r,s))
#[[1]]
#[1] "xy1234" "xy567"
Not what I want because it returns the substrings matching the entire pattern.
Second try: regexec:
regmatches(s,regexec("xy(\\d+)",s))
#[[1]]
#[1] "xy1234" "1234"
Not what I want because it returns only the first occurence of a matching for the entire pattern and the capture group.
If there was a gregexec function, extending regexec as gregexpr extends regexpr, my problem would be solved.
So the question is: how to retrieve all substrings (or indices that can be passed to regmatches as in the examples above) matching capture groups in an arbitrary regular expression?
Note: the pattern for r given above is just a silly example, it must remain arbitrary.
Not sure about doing this in base, but here's a package for your needs:
library(stringr)
str_match_all(s, r)
#[[1]]
# [,1] [,2]
#[1,] "xy1234" "1234"
#[2,] "xy567" "567"
Many stringr functions also have parallels in base R, so you can also achieve this without using stringr.
For instance, here's a simplified version of how the above works, using base R:
sapply(regmatches(s,gregexpr(r,s))[[1]], function(m) regmatches(m,regexec(r,m)))