stringr 1.2.0

API changes

• str_match_all() now returns NA if an optional group doesn't match
  (previously it returned ""). This is more consistent with str_match()
  and other match failures (#134).

New features

• In str_replace(), replacement can now be a function that is called once
  for each match and who's return value is used to replace the match.
• New str_which() mimics grep() (#129).
• A new vignette (vignette("regular-expressions")) describes the
  details of the regular expressions supported by stringr.
  The main vignette (vignette("stringr")) has been updated to
  give a high-level overview of the package.

Minor improvements and bug fixes

• str_order() and str_sort() gain explicit numeric argument for sorting
  mixed numbers and strings.
• str_replace_all() now throws an error if replacement is not a character
  vector. If replacement is NA_character_ it replaces the complete string
  with replaces with NA (#124).
• All functions that take a locale (e.g. str_to_lower() and str_sort())
  default to "en" (English) to ensure that the default is consistent across
  platforms.