regex - Extracting a string between other two strings in R

Question

I am trying to find a simple way to extract an unknown substring (could be anything) that appear between two known substrings. For example, I have a string:

a<-" anything goes here, STR1 GET_ME STR2, anything goes here"

I need to extract the string GET_ME which is between STR1 and STR2 (without the white spaces).

I am trying str_extract(a, "STR1 (.+) STR2"), but I am getting the entire match

[1] "STR1 GET_ME STR2"

I can of course strip the known strings, to isolate the substring I need, but I think there should be a cleaner way to do it by using a correct regular expression.

Answer 1

You may use str_match with STR1 (.*?) STR2 (note the spaces are "meaningful", if you want to just match anything in between STR1 and STR2 use STR1(.*?)STR2, or use STR1\s*(.*?)\s*STR2 to trim the value you need). If you have multiple occurrences, use str_match_all.

Also, if you need to match strings that span across line breaks/newlines add (?s) at the start of the pattern: (?s)STR1(.*?)STR2 / (?s)STR1\s*(.*?)\s*STR2.

library(stringr)
a <- " anything goes here, STR1 GET_ME STR2, anything goes here"
res <- str_match(a, "STR1\s*(.*?)\s*STR2")
res[,2]
[1] "GET_ME"

Another way using base R regexec (to get the first match):

test <- " anything goes here, STR1 GET_ME STR2, anything goes here STR1 GET_ME2 STR2"
pattern <- "STR1\s*(.*?)\s*STR2"
result <- regmatches(test, regexec(pattern, test))
result[[1]][2]
[1] "GET_ME"