We can use the same technique for extracting the file extension as we used for extracting the whole filename in Recipe 8.23.

The only difference is in how we handle dots. The regex in Recipe 8.23 does not include any dots. The negated character class in that regex will simply match any dots that happen to be in the filename.

A file extension must begin with a dot. Thus, we add ‹\.› to match a literal dot at the start of the regex.

Filenames such as Version 2.0.txt may contain multiple dots. The last dot is the one that delimits the extension from the filename. The extension itself should not contain any dots. We specify this in the regex by putting a dot inside the character class. The dot is simply a literal character inside character classes, so we don’t need to escape it. The ‹$› anchor at the end of the regex makes sure we match .txt instead of .0.

If the string ends with a backslash, or with a filename that doesn’t include any dots, the regex won’t match at all. When it does match, it will match the extension, including the dot that delimits the extension and the filename.

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