Talk:Extract file extension

From Rosetta Code

a new contributor

Hi, I was a quiet reader but decided to contribute now. That is my first every wiki-like article so please excuse my mistakes and kindly tell how to improve. :)

Hi, First thanks for taking the trouble to lurk and try and get the hang of things before making your first post. It is appreciated :-)
I re-wrote bits for style and trying to be a little more exacting in the definition. I also added two more tests that fit the revised definition, but the existing C# might need revising and extra output. I also stuck nowiki tags around the example link.
What do you think? --Paddy3118 (talk) 10:22, 4 May 2015 (UTC)

Thank you Paddy. I did not think about your test cases. I believe the underscore one is legal according to this site: Yes, you are right. My C# code needs to be updated. I will try to do that, when I find time.

definition of a legal file extension

If the   file extension   appears   after   the last period (using the first example), then the file extension should be   jpg,   and not   .jpg   (just a nitpick).   Also note that   .jpg   contains a non-alphanumeric character, namely the period.   In various documents that I read, the file extension is to the   right   of the last period (with other caveats, of course), and does not include the period.   But other sites do include the period, so ... -- Gerard Schildberger (talk) 23:26, 4 May 2015 (UTC)

Also note that the legality of what is a legal file extension depends on the operating system.   Windows/95 for instance, greatly expanded what is legal (including the use of additional characters, including a blank, as well as lowercase letters).   And I can't vouch for the various flavors of *NIX. -- Gerard Schildberger (talk) 23:26, 4 May 2015 (UTC)

Also, I would use the word   digits   (or better yet,   decimal digits)   instead of   numbers   when defining a legal file extension (for this Rosetta Code task). -- Gerard Schildberger (talk) 23:42, 4 May 2015 (UTC)

It wouldn't hurt to define what a   letter   is;   I assume you meant lower and uppercase versions of the Latin (Roman) alphabet.   If so, specifically mention them.   I don't feel comfortable assuming what was stated. -- Gerard Schildberger (talk) 23:42, 4 May 2015 (UTC)

Numerals   is a better word to use instead of   numbers   in this case. -- Gerard Schildberger (talk) 03:25, 5 May 2015 (UTC)

  (pi)   is a number,   so is  +1.5   and   -6.   -- Gerard Schildberger (talk) 23:42, 4 May 2015 (UTC)

Another also:

The 2nd part of the definition of a filename extension states that:

  *  consists of a period, followed by one or more ASCII letters or digits (A-Z, a-z, 0-9)

I would argue that the filename  


fits that requirement as the numeral   5   follows the period.

Therefore, the following would be more apt:

  *  consists of a period, followed  solely  by one or more ASCII letters or digits (A-Z, a-z, 0-9)

which would exclude any extraneous characters.

And, yes, I know that one of the test cases (examples) shows this, but examples shouldn't be used as definitions. -- Gerard Schildberger (talk) 17:40, 31 August 2016 (UTC)

Updated. --Smls (talk) 06:19, 1 September 2016 (UTC)

Another case I find useful is "" => "so", and always returning lowercase results. Just sayin Pete Lomax (talk) 12:18, 9 September 2016 (UTC)

In Unix and Unix-like environments


is a hidden file or directory and not a file extension.

Updated:--Garak (talk) 12:34, 21 December 2017 (UTC):
That's why the specification says "For the purposes of this task", and the task allows showing a built-in standard library function that behaves differently. --Smls (talk) 16:32, 21 December 2017 (UTC)

Use Libraries not Footguns

Practicing programmers should strongly prefer to use existing filesystem APIs to extract file extensions.


1. Doing this correctly depends on the operating system (OS).

2. Such OS APIs exist for a reason! Use them!

3. Generally speaking, handling strings correctly is harder than many programmers realize. There are many gotchas in doing this correctly.

4. You need to be smart about encodings and locales.

Therefore, for the sake of being *responsible* about what information Rosetta Code provides, it should do the following:

A. Make it abundantly clear that ad-hoc solutions are *not* applicable for real-world usage.

B. Point readers to the relevant APIs in their languages, operating systems, and libraries.

--Krebsikl (talk) 15:27, 7 August 2023 (UTC)

It would help if you could name even just one of them, and at least one example that c/would go horribly wrong. I was not aware of any suitable OS APIs for this, perhaps you meant PathCchFindExtension, but that is not capable of solving this task. I would agree that handling strings is much harder than it should be, and quite ridiculously so in most popular (/terrible) programming languages, and via most OS APIs. If some task on rc is not suitable for real-world use, then (like you just have) you need to start a debate on that task's discussion page, and (possibly with a limited few exceptions for complexity or security reasons) try and lick the task into something that is useful, since comparing programming languages (the whole purpose of this site) for completely unusable tasks is, well, all rather pointless. --Petelomax (talk) 15:48, 9 August 2023 (UTC)