You have a string that holds a (syntactically) valid path to a file or folder on a Windows PC or network. You want to extract the drive letter, if any, from the path. For example, you want to extract c from c:folderfile.ext.

^([a-z]):

Extracting the drive letter from a string known to hold a valid path is trivial, even if you don’t know whether the path actually starts with a drive letter. The path could be a relative path or a UNC path.

Colons are invalid characters in Windows paths, except to delimit the drive letter. Thus, if we have a letter followed by a colon at the start of the string, we know the letter is the drive letter.

The anchor ‹^› matches at the start of the string (Recipe 2.5). The fact that the caret also matches at embedded line breaks in Ruby doesn’t matter, because valid Windows paths don’t include line breaks. The character class ‹[a-z]› matches a single letter (Recipe 2.3). We place the character class between a pair of parentheses (which form a capturing group) so you can get the drive letter without the literal colon that is also matched by the regular expression. We add the colon to the regular expression to make sure we’re extracting the drive letter, rather than the first letter in a relative path.

Recipe 2.9 tells you all about capturing groups.

See Recipe 3.9 to learn how to retrieve text matched by capturing groups in your favorite programming language.

Follow Recipe 8.19 if you don’t know in advance that your string holds a valid Windows path.