How to extract everything until first occurrence of pattern

Question

I'm trying to use the stringr package in R to extract everything from a string up until the first occurrence of an underscore.

What I've tried

str_extract("L0_123_abc", ".+?(?<=_)")
> "L0_"

Close but no cigar. How do I get this one? Also, Ideally I'd like something that's easy to extend so that I can get the information in between the 1st and 2nd underscore and get the information after the 2nd underscore.

Answer 1

To get L0, you may use

> library(stringr)
> str_extract("L0_123_abc", "[^_]+")
[1] "L0"

The [^_]+ matches 1 or more chars other than _.

Also, you may split the string with _:

x <- str_split("L0_123_abc", fixed("_"))
> x
[[1]]
[1] "L0"  "123" "abc"

This way, you will have all the substrings you need.

The same can be achieved with

> str_extract_all("L0_123_abc", "[^_]+")
[[1]]
[1] "L0"  "123" "abc"

Answer 2

The regex lookaround should be

str_extract("L0_123_abc", ".+?(?=_)")
#[1] "L0"

Answer 3

Using gsub...

gsub("(.+?)(\\_.*)", "\\1", "L0_123_abc")

Answer 4

You can use sub from base using _.* taking everything starting from _.

sub("_.*", "", "L0_123_abc")
#[1] "L0"

Or using [^_] what is everything but not _.

sub("([^_]*).*", "\\1", "L0_123_abc")
#[1] "L0"

or using substr with regexpr.

substr("L0_123_abc", 1, regexpr("_", "L0_123_abc")-1)
#substr("L0_123_abc", 1, regexpr("_", "L0_123_abc", fixed=TRUE)-1) #More performant alternative
#[1] "L0"