Revise Windows implementation of splitdrive (JuliaLang#42204)

This PR makes three improvements to the Windows `splitdrive`
implementation:

1. The matched regex is split into pieces and annotated.
2. Forward and backward slashes are considered equivalent. This fixes
   JuliaLang#38492.
3. The patterns in the regex are reordered so that long UNC paths and
   long drive letters are once more recognized. This has been broken
   since JuliaLang#19695 (Julia 0.5).

Co-authored-by: Jameson Nash <[email protected]>

base/path.jl

@@ -34,8 +34,25 @@ elseif Sys.iswindows()
     const path_dir_splitter = r"^(.*?)([/\\]+)([^/\\]*)$"sa
     const path_ext_splitter = r"^((?:.*[/\\])?(?:\.|[^/\\\.])[^/\\]*?)(\.[^/\\\.]*|)$"sa
 
+    const splitdrive_re = let
+        # Slash in either direction.
+        S = raw"[\\/]"
+        # Not a slash in either direction.
+        N = raw"[^\\/]"
+        # Drive letter, e.g. `C:`
+        drive = "$(N)+:"
+        # UNC path, e.g. `\\server\share`
+        unc = "$(S)$(S)$(N)+$(S)$(N)+"
+        # Long drive letter, e.g. `\\?\C:`
+        long_drive = "$(S)$(S)\\?$(S)$(drive)"
+        # Long UNC path, e.g. `\\?\UNC\server\share`
+        long_unc = "$(S)$(S)\\?$(S)UNC$(S)$(N)+$(S)$(N)+"
+        # Need to match the long patterns first so they get priority.
+        Regex("^($long_unc|$long_drive|$unc|$drive|)(.*)\$", "sa")
+    end
+
     function splitdrive(path::String)
-        m = match(r"^([^\\]+:|\\\\[^\\]+\\[^\\]+|\\\\\?\\UNC\\[^\\]+\\[^\\]+|\\\\\?\\[^\\]+:|)(.*)$"sa, path)::AbstractMatch
+        m = match(splitdrive_re, path)::AbstractMatch
         String(something(m.captures[1])), String(something(m.captures[2]))
     end
 else