Regex get string between intervals underscores-CodePudding

I've seen a lot of similar questions, but I wasn't able to get the desired output.

I have a string means_variab_textimput_x2_200.txt and I want to catch ONLY what is between the third and fourth underscores: textimput

I'm using R, stringr, I've tried many things, but none solved the issue:

my_string <- "means_variab_textimput_x2_200.txt"

str_extract(my_string, '[_]*[^_]*[_]*[^_]*[_]*[^_]*')
"means_variab_textimput"

str_extract(my_string, '^(?:([^_] )_){4}')
"means_variab_textimput_x2_"
str_extract(my_string, '[_]*[^_]*[_]*[^_]*[_]*[^_]*\\.') ## the closer I got was this
"_textimput_x2_200."

Any ideas? Ps: I'm VERY new to Regex, so details would be much appreciated :)
additional question: can I also get only a "part" of the word? let's say, instead of textimput only text but without counting the words? It would be good to know both possibilities
this this one this one were helpful, but I couldn't get the final expected results. Thanks in advance.

CodePudding user response：

stringr uses ICU based regular expressions. Therefore, an option would be to use regex lookarounds, but here the length is not fixed, thus (?<= wouldn't work. Another option is to either remove the substrings with str_remove or use str_replace to match and capture the third word which doesn't have the _ ([^_] ) and replace with the backreference (\\1) of the captured word

library(stringr)
str_replace(my_string, "^[^_] _[^_] _([^_] )_.*", "\\1") 
[1] "textimput"

If we need only the substring

str_replace(my_string, "^[^_] _[^_] _([^_]{4}).*", "\\1") 
[1] "text"

In base R, it is easier with strsplit and get the third word with indexing

strsplit(my_string, "_")[[1]][3]
# [1] "textimput"

Or use perl = TRUE in regexpr

regmatches(my_string, regexpr("^([^_] _){2}\\K[^_] ", my_string, perl = TRUE))
# [1] "textimput"

For the substring

regmatches(my_string, regexpr("^([^_] _){2}\\K[^_]{4}", my_string, perl = TRUE))
[1] "text"

CodePudding user response：

Following up on question asked in comment about restricting the size of the extracted word, this can easily be achieved using quantification. If, for example, you want to extract only the first 4 letters:

sub("[^_] _[^_] _([^_]{4}).*$", "\\1", my_string)
[1] "text"