ASP.net MVC regular expression in view model for filename validation

Question

I've read a few questions that answer this, and I understand the regular expression I'm required to use, however actually applying it in MVC is where I stumble. I will also preface by saying I am terrible at regular expressions so far.

I'm writing a file upload application in MVC and I want to apply standard windows filename validation. \/:*?"<>| are invalid characters anywhere in the name.

My View Model for this is setup like so, using a different regex I found:

    public class FileRenameModel
{
    [RegularExpression(@"^[\w\-. ]+$", ErrorMessage="A filename cannot contain \\ / : * ? \" < > |")]
    [Required]
    public string Filename { get; set; }
    [Required]
    public int FileID      { get; set; }
}

Whenever I try to change the regex to @"^[\\/:?"<>|]+$ the " in the middle kills it and throws an error. I haven't figured out how to properly escape it so that I can include it in the string. When I use the regex without the " it tells me any string I put into the textbox fails. Am I using the ^ incorrectly?

Paul · Accepted Answer · 2013-05-02 18:30:09Z

4

Use double "" to escape quotes after starting a string with @.

To search for anything except you'd want to insert an additional ^ inside the brackets to create an except for match: @"^[^\\/:?""<>|]+$" Keep the ^ at the beginning as well to match the start of line.

Having said that, keep in mind for validation that browsers handle file names differently. Some older browsers sent a path along with the filename, that might break your validation for a legitimate file.

edited May 2, 2013 at 18:30

answered May 2, 2013 at 18:03

Paul

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4 Comments

MacSalty Over a year ago

This part has helped me with the ", thank you. However I'm still unfortunately stuck on the logic.

MacSalty Over a year ago

Thank you Paul. I ended up trying this just before checking your comment and it worked, but now I have an explanation as to why to do it that way. I will have to check for my validation breaking a legitimate file name.

Paul Over a year ago

Yeah, USUALLY this will work, but like I said some browsers send either the actual path or prepend some weirdness like ://. If you can, it might be best just to sanitize the filename in the request handler by removing any of the characters returned by System.IO.Path.GetInvalidPathChars().

MacSalty Over a year ago

I totally just realized that I've actually implemented sanitation. Not enough coffee this morning. Thanks again, it's a good learning experience for me.

Nicholas Carey · Accepted Answer · 2013-05-02 19:06:44Z

This regular expression should match a more-than-sufficient subset of valid NTFS filenames (bear in mind that an NTFS file name may contain pretty much any Unicode character.)

Regex rxValidFileName = new Regex(@"^[[\p{IsBasicLatin}\p{IsLatin-1Supplement}\p{IsLatinExtended-A}\p{IsLatinExtended-B}]-[\p{C}<>:""/\|?*]]+$" , RegexOptions.IgnorePatternWhitespace|RegexOptions.IgnoreCase);

What this matches:

start-of-line, followed by
1 or more of
- any basic latin, latin-1 supplement, or latin extended-A or -B character, unless...
- unless it's a C0 or C1 control character or one of the characters otherwise disallowed by NTFS — <>:"/\|?*
terminated by end-of-line.

Note that this matches a file name, not a file path. A file path is more complicated, since it's got a grammar to it, something like this, in crude ABNF:

filepath : relative-path
         | absolute-path
         | drivespecifier (relative-path|absolute-path)?
         | unc-share (absolute-path)?
         ;

relative-path : filename ( directory-separator filename? )*

absolute-path : directory-separator ( filename? directory-separator )*

directory-separator : [/\]

drivespecifier : [a-zA-Z] ":"

unc-share : "\\" filename "\" filename absolute-path?

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