We’ve made only one change to the regular expressions for drive letter paths, compared to the ones in the previous recipe. We’ve added three capturing groups that you can use to retrieve the various parts of the path: ‹drive›, ‹folder›, and ‹file›. You can use these names if your regex flavor supports named capture (Recipe 2.11). If not, you’ll have to reference the capturing groups by their numbers: 1, 2, and 3. See Recipe 3.9 to learn how to get the text matched by named and/or numbered groups in your favorite programming language.

We’ve added the same three capturing groups to the regexes for UNC paths.

Things get a bit more complicated if we also want to allow relative paths. In the previous recipe, we could just add a third alternative to the drive part of the regex to match the start of the relative path. We can’t do that here. In case of a relative path, the capturing group for the drive should remain empty.

Instead, the literal backslash that was after the capturing group for the drives in the regex in the “drive letter and UNC paths” section is now moved into that capturing group. We add it to the end of the alternatives for the drive letter and the network share. We add a third alternative with an optional backslash for relative paths that may or may not begin with a backslash. Because the third alternative is optional, the whole group for the drive is essentially optional.

The resulting regular expression correctly matches all Windows paths. The problem is that by making the drive part optional, we now have a regex in which everything is optional. The folder and file parts were already optional in the regexes that support absolute paths only. In other words: our regular expression will match the empty string.

If we want to make sure the regex doesn’t match empty strings, we’d have to add additional alternatives to deal with relative paths that specify a folder (in which case the filename is optional), and relative paths that don’t specify a folder (in which case the filename is mandatory):

\A
(?:
   (?<drive>[a-z]:|\\\\[a-z0-9_.$●-]+\\[a-z0-9_.$●-]+)\\
   (?<folder>(?:[^\\/:*?"<>|\r\n]+\\)*)
   (?<file>[^\\/:*?"<>|\r\n]*)
|  (?<relativefolder>\\?(?:[^\\/:*?"<>|\r\n]+\\)+)
   (?<file2>[^\\/:*?"<>|\r\n]*)
|  (?<relativefile>[^\\/:*?"<>|\r\n]+)
)
\Z

\A
(?:
   (?P<drive>[a-z]:|\\\\[a-z0-9_.$●-]+\\[a-z0-9_.$●-]+)\\
   (?P<folder>(?:[^\\/:*?"<>|\r\n]+\\)*)
   (?P<file>[^\\/:*?"<>|\r\n]*)
|  (?P<relativefolder>\\?(?:[^\\/:*?"<>|\r\n]+\\)+)
   (?P<file2>[^\\/:*?"<>|\r\n]*)
|  (?P<relativefile>[^\\/:*?"<>|\r\n]+)
)
\Z

\A
(?:
   ([a-z]:|\\\\[a-z0-9_.$●-]+\\[a-z0-9_.$●-]+)\\
   ((?:[^\\/:*?"<>|\r\n]+\\)*)
   ([^\\/:*?"<>|\r\n]*)
|  (\\?(?:[^\\/:*?"<>|\r\n]+\\)+)
   ([^\\/:*?"<>|\r\n]*)
|  ([^\\/:*?"<>|\r\n]+)
)
\Z

^(?:([a-z]:|\\\\[a-z0-9_.$●-]+\\[a-z0-9_.$●-]+)\\↵
((?:[^\\/:*?"<>|\r\n]+\\)*)([^\\/:*?"<>|\r\n]*)|(\\?(?:[^\\/:*?"<>|↵
\r\n]+\\)+)([^\\/:*?"<>|\r\n]*)|([^\\/:*?"<>|\r\n]+))$

\A
(?:
   (?<drive>[a-z]:|\\\\[a-z0-9_.$●-]+\\[a-z0-9_.$●-]+)\\
   (?<folder>(?:[^\\/:*?"<>|\r\n]+\\)*)
   (?<file>[^\\/:*?"<>|\r\n]*)
|  (?<folder>\\?(?:[^\\/:*?"<>|\r\n]+\\)+)
   (?<file>[^\\/:*?"<>|\r\n]*)
|  (?<file>[^\\/:*?"<>|\r\n]+)
)
\Z