R-根据模式处理字符串

Question

This is the name of a file that I have on R:

> lst.files[1]
[1] "clt_Amon_CanESM2_rcp45_185001-230012.nc"

What I need to do is capture just the part until the 4th underscore (including), so it would be something like this:

clt_Amon_CanESM2_rcp45_

How can I get this in R?

Answer 1

Using the qdap package, you can do the following.

x <- "clt_Amon_CanESM2_rcp45_185001-230012.nc"

library(qdap)
beg2char(x, "_", 4, include = TRUE)
# [1] "clt_Amon_CanESM2_rcp45_"

Answer 2

If you know you always have (at least) four underscores, then you could do something like this:

regmatches(lst, regexec(".*_.*_.*_.*_", lst.files[1]))[[1]]
# [1] "clt_Amon_CanESM2_rcp45_"

If potentially not always four, but no underscores in the second part, you could do something like this:

regmatches(lst, regexec(".*_", lst.files[1]))[[1]]
# [1] "clt_Amon_CanESM2_rcp45_"

This doesn't require any extra package, just base R.

Answer 3

We can also capture the repeating patterns as a group using sub . We match one more more characters from the beginning ( ^ ) of the string that is not an underscore ( [^_]+ ) followed by an underscore ( \\\\_ ) which is repeated 4 times ( {4} ), capture that as a group by wrapping with parentheses followed by zero or more characters ( .* ). We replace it with the capture group ( \\\\1 ) to get the expected output.

sub('^(([^_]+\\_){4}).*', '\\1', str1)
#[1] "clt_Amon_CanESM2_rcp45_"

data

str1 <-  "clt_Amon_CanESM2_rcp45_185001-230012.nc"