What's a robust way to insert another extension into a filename?

Question

I'd like to add an extension to a filename before the file extension, otherwise leaving the given filename the same. In particular, absolute filenames should stay absolute, and relative filenames should stay relative. Examples: suppose the piece I want to insert is ".123", then

• /home/me/dir/foo.txt becomes /home/me/dir/foo.123.txt.
• xyz.csv becomes xyz.123.csv
• foobar becomes foobar.123 (just append in case of no extension)

I don't expect to be dealing with filenames that already have multiple extensions, but just in case, the desired behavior is

• nomnom.tar.gz becomes nomnom.123.tar.gz

I can ensure that the filename does not end with a slash.

The obvious way to do this is by concatenating the directory name, the file base name, the new piece, and the extension:

insertPiece[fn_, piece_] := FileNameJoin[{
      DirectoryName[fn],
      StringJoin[{FileBaseName[fn], piece, ".", FileExtension[fn]}]
}]

but is there some corner case I missed in which this wouldn't work? Is there a more efficient or more elegant way to do it?

Answer 1

Here's my non-regex solution (but it does use a "String Pattern", which is equivalent to regex). I think it is robust.

insertExtension[fn_String, piece_String] := 
 Module[{split = FileNameSplit[fn], temp},
  temp = Insert[StringSplit[Last[split], "."], piece, 2];
  temp = StringJoin[Riffle[temp, "."]];
  FileNameJoin[Append[Most[split], temp]]]

Test:

In[]:= insertExtension["/home/me.em/dir.ab/nomnom.tar.gz", "123"]

Out[]= "/home/me.em/dir.ab/nomnom.123.tar.gz"

Answer 2

String manipulation like this is probably done best using regular expressions. If you're not familiar with these, you may want to have a look at the Wikipedia article or the Mathematica tutorial on them first.

The code given below defines a pattern regex that matches an extension of the type .abcDeF... at the end of a string (that's accomplished by the $ sign). If that pattern does not match, then the extension is simply added; if the pattern matches, the extension is put in between the rest of the string and the match. Here's the code for that:

appendExtension[filename_, extension_] := Block[{regex},
    regex = RegularExpression["\\.[a-z]+$"];
    If[Length@StringCases[filename, regex] == 0,
        filename <> "." <> extension,
        StringReplace[filename, regex -> ("." <> extension <> "$0")]
    ]
];
appendExtension["/home/me/dir/foo.txt", "123"]
appendExtension["xyz.csv", "123"]
appendExtension["foobar", "123"]

Output:

"/home/me/dir/foo.123.txt"
"xyz.123.csv"
"foobar.123"

A few remarks:

• I'm not sure how you want filenames like foo.bar.baz to be handled. The code above replaces this by foo.bar.123.baz, i.e. takes only the last extension into account. If you want the new one inserted as the leftmost one, change the regular expression to (\\.[a-z]+)+$, and the replacement will yield foo.123.bar.baz.
• The regex doesn't include upper case extensions or digits. To add these, change [a-z] inside the regex to [a-zA-Z0-9].