Regular expression for valid filename

Question

I already gone through some question in StackOverflow regarding this but nothing helped much in my case.

I want to restrict the user to provide a filename that shoul

Answer 1

I may be saying something stupid here, but it seems to me that these answers aren't correct. Firstly, are we talking Linux or Windows here (or another OS)?

Secondly, in Windows it is (I believe) perfectly legitimate to include a "$" in a filename, not to mention Unicode in general. It certainly seems possible.

I tried to get a definitive source on this... and ending up at the Wikip Filename page: in particular the section "Reserved characters and words" seems relevant: and these are, clearly, a list of things which you are NOT allowed to put in.

I'm in the Java world. And I naturally assumed that Apache Commons would have something like validateFilename, maybe in FilenameUtils... but it appears not (if it had done, this would still be potentially useful to C# programmers, as the code is usually pretty easy to understand, and could therefore be translated). I did do an experiment, though, using the method normalize: to my disappointment it allowed perfectly invalid characters (?, etc.) to "pass".

The part of the Wikip Filename page referenced above shows that this question depends on the OS you're using... but it should be possible to concoct some simple regex for Linux and Windows at least.

Then I found a Java way (at least):

Path path = java.nio.file.FileSystems.getDefault().getPath( 'bobb??::mouse.blip' );

output:

java.nio.file.InvalidPathException: Illegal char at index 4: bobb??::mouse.blip

... presumably different FileSystem objects will have different validation rules

Answer 2

In case someone else needs to validate filenames (including Windows reserved words and such), here's a full expression: \A(?!(?:COM[0-9]|CON|LPT[0-9]|NUL|PRN|AUX|com[0-9]|con|lpt[0-9]|nul|prn|aux)|[\s\.])[^\\\/:*"?<>|]{1,254}\z

Extended expression (don't allow filenames starting with 2 dots, don't allow filenames ending in dots or whitespace):

\A(?!(?:COM[0-9]|CON|LPT[0-9]|NUL|PRN|AUX|com[0-9]|con|lpt[0-9]|nul|prn|aux)|\s|[\.]{2,})[^\\\/:*"?<>|]{1,254}(?<![\s\.])\z

Edit: For the interested, here's a link to Windows file naming conventions: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx

Answer 3

Copied from @Engineer for future reference as the dot was not escaped (as it should) in the most voted answer.

This is the correct expression:

string regex = @"^[\w\-\. ]+$";

Answer 4

I've just created this. It prevents two dots and dot at end and beginning. It doesn't allow any two dots though.

^([a-zA-Z0-9_]+)\.(?!\.)([a-zA-Z0-9]{1,5})(?<!\.)$