How to extract a certain part of a string in R using regular expressions

Question

How to convert the following string in R :

this_isastring_12(=32)

so that only the following is kept

isastring_12

Eg

f('this_isastring_12(=32)') returns 'isastring_12'

This should work on other strings with a similar structure, but different characters

Another example with a different string of similar structure

f('something_here_3(=1)') returns 'here_3'

What is the logic of conversion ? Keep everything after....? Or remove everything after.. ?Or something else ? — Ronak Shah
– Ronak Shah, Commented Oct 8, 2019 at 4:23
so sub(".*(isastring_12).*", "\\1", "this_isastring_12(=32)") would work ? — Ronak Shah
– Ronak Shah, Commented Oct 8, 2019 at 4:27
yes but it wouldn't generalise, I guess if the parens were (.*?) it might work? — baxx
– baxx, Commented Oct 8, 2019 at 4:28
that is what I mean when I asked you for the logic of conversion ? Do you want to keep everything after first underscore till an opening bracket is found or is there some other logic involved ? — Ronak Shah
– Ronak Shah, Commented Oct 8, 2019 at 4:30

Ronak Shah · Accepted Answer · 2019-10-08 04:35:22Z

1

We can use sub to extract everything from first underscore to opening round bracket in the text.

sub(".*?_(.*)\\(.*", "\\1", x)
#[1] "isastring_12" "here_3"       "string_4"

where x is

x <- c("this_isastring_12(=32)", "something_here_3(=1)", "another_string_4(=1)")

answered Oct 8, 2019 at 4:35

Ronak Shah

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moodymudskipper · Accepted Answer · 2019-10-08 15:43:12Z

You could use the package unglue.

Borrowing Ronak's data :

x <- c("this_isastring_12(=32)", "something_here_3(=1)", "another_string_4(=1)")
library(unglue)
unglue_vec(x, "{=.*?}_{res}({=.*?})")
#> [1] "isastring_12" "here_3"       "string_4"

{=.*?} matches anything until what's next is matched, but doesn't extract anything because there's no lhs to the equality
{res}, where the name res could be replaced by anything, matches anything, and extracts it
outside of curly braces, no need to escape characters
unglue_vec() returns an atomic vector of the matches

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