Extract substring in R using grepl

I have a table with a string column formatted like this

abcdWorkstart.csv
abcdWorkcomplete.csv


And I would like to extract the last word in that filename. So I think the beginning pattern would be the word "Work" and ending pattern would be ".csv". I wrote something using grepl but not working.

grepl("Work*.csv", data$filename)


Basically I want to extract whatever between Work and .csv

desired outcome:

start
complete


4 Answers
4

Just as an alternative way, remove everything you don't want.

x <- c("abcdWorkstart.csv", "abcdWorkcomplete.csv")

gsub("^.*Work|\.csv$", "", x)
#[1] "start" "complete"


please note:
I have to use gsub. Because I first remove ^.*Work then \.csv$.

gsub

^.*Work

\.csv$

For [\s\S] or \d\D ... (does not work with [g]?sub)

[\s\S]

\d\D

https://regex101.com/r/wFgkgG/1

Works with akruns approach:

regmatches(v1, regexpr("(?<=Work)[\s\S]+(?=[.]csv)", v1, perl = T))

regmatches(v1, regexpr("(?<=Work)[\s\S]+(?=[.]csv)", v1, perl = T))

str1<-
'12
.2
12'

gsub("[^.]","m",str1,perl=T)
gsub(".","m",str1,perl=T)
gsub(".","m",str1,perl=F)


. matches also n when using the R engine.

.

n

I think you need sub or gsub (substitute/extract) instead of grepl (find if match exists). Note that when not found, it will return the entire string unmodified:

sub

gsub

grepl

fn <- c('abcdWorkstart.csv', 'abcdWorkcomplete.csv', 'abcdNothing.csv')
out <- sub(".*Work(.*)\.csv$", "\1", fn)
out
# [1] "start" "complete" "abcdNothing.csv"


You can work around this by filtering out the unchanged ones:

out[ out != fn ]
# [1] "start" "complete"


Or marking them invalid with NA (or something else):

NA

out[ out == fn ] <- NA
out
# [1] "start" "complete" NA


With str_extract from stringr. This uses positive lookarounds to match any character one or more times (.+) between "Work" and ".csv":

str_extract

stringr

x <- c("abcdWorkstart.csv", "abcdWorkcomplete.csv")

library(stringr)
str_extract(x, "(?<=Work).+(?=\.csv)")
# [1] "start" "complete"


Here is an option using regmatches/regexpr from base R. Using a regex lookaround to match all characters that are not a . after the string 'Work', extract with regmatches

regmatches/regexpr

base R

.

regmatches

regmatches(v1, regexpr("(?<=Work)[^.]+(?=[.]csv)", v1, perl = TRUE))
#[1] "start" "complete"


v1 <- c('abcdWorkstart.csv', 'abcdWorkcomplete.csv', 'abcdNothing.csv')


"(?<=Work).*(?=.csv)"

regmatches/regexpr

[\s\S]

.

[.]

\.

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