str_match and str_match_all behave inconsistently when groups are missing

[kindlychung]

Here is an example:

  string = "-3e9, 9E-2"
  pattern <- "([+-]?)(\\d+)(\\.\\d+)?[eE]([+-]?)(\\d+)"
  stringr::str_match_all(string = string, pattern = pattern)
  stringr::str_match(string = string, pattern = pattern)

str_match reports NA, while str_match_all reports an empty string.

[hadley]

This seems to be the correct fix, as a while back I made str_match consistent with str_extract etc.