date format split

library(magrittr)
strings = c("03Jan-05Jan201602Jul-04Jul201602Dec-04Dec2016", "06Jan-09Jan2016January 2017",
  "May 201607Jan-08Jan2016", "07Jan-10Jan2016MAY 201706Jan-09Jan2018")
lapply(strings, function(x){
  m = gregexpr('\\d{4}', x)
  loc = c(0, m[[1]] + 3)
  out = vector('character', length(m))
  for (i in 2:length(loc))
    out[i-1] = substr(x, (loc[i-1]+1), loc[i])
  out
})