regex - list.files pattern argument in R, extended regular expression use

Question

I run

  dir.create('./junk_data')
  file.create(paste('./junk_data/QWE',01:12,01:31,2005:2015,'.3',sep=''))
  file.create(paste('./junk_data/RTY',01:12,01:31,2005:2015,'.3',sep=''))

and want to list all the files that begin with QWE and end with 2011.3. I tried

list.files('./junk_data/',pattern='QWE....2011.3',full.names=T)

and

list.files('./junk_data/',pattern='QWE....2011.3',full.names=T,perl=T)

but I guess '.' doesn't mean one what I think, as I get none of the files I want.

I tried a few tutorials on regex, but no joy.

Answer 1

As Arun showed in his example, a dot usually means "match any character", so to match a dot you need to escape it: \.. You can create the pattern most easily with glob2rx, which uses * as a wildcard and matches other characters as though they are fixed.

glob2rx("QWE*2011.3")   #"^QWE.*2011\.3$"
list.files("./junk_data/", pattern = glob2rx("QWE*2011.3"), full.names = TRUE)