问题
I try to list all files in a directory that do not start with "Camera1", but end with ".png". For doing so, I am using a regular expression in list.files in R. To exclude "Camera1", I tried to use a negative lookahead, but it doesn't work. Where is my mistake? ;)
list.files(pathToDirectory, pattern = "^(?!Camera1).*\\.png")
I get the error: invalid 'pattern' regular expression
Thanks in advance :)
回答1:
Looks like the default engine doesn't like lookarounds, so you need to use Perl. This works:
dat <- c("Camera1.png", "Camera2.png", "hello.png", "boo")
grep("^(?!Camera1).*\\.png", dat, value=T, perl=T)
# [1] "Camera2.png" "hello.png"
But this doesn't:
grep("^(?!Camera1).*\\.png", dat, value=T)
# invalid regular expression '(?<!Camera1)\.png', reason 'Invalid regexp'
So, to do what you what you want:
grep("(?<!Camera1)\\.png", list.files(), perl=T, value=T)
来源:https://stackoverflow.com/questions/30535960/negative-look-ahead-assertion-in-list-files-in-r