Invalid characters for Windows filenames

ntfs-filenames.txt

Information from https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file :

    Use any character in the current code page for a name, including Unicode
    characters and characters in the extended character set (128–255), except
    for the following:

    - The following reserved characters:

        < (less than)
        > (greater than)
        : (colon)
        " (double quote)
        / (forward slash)
        \ (backslash)
        | (vertical bar or pipe)
        ? (question mark)
        * (asterisk)

    - Integer value zero, sometimes referred to as the ASCII NUL character.
    - Characters whose integer representations are in the range from 1 through
      31, except for alternate data streams where these characters are
      allowed. For more information about file streams, see File Streams.
    - Any other character that the target file system does not allow.
    
    - Do not use the following reserved names for the name of a file:
        
        CON, PRN, AUX, NUL, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8,
        COM9, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9.
        
        Also avoid these names followed immediately by an extension; for
        example, NUL.txt is not recommended.

    - Do not end a file or directory name with a space or a period. Although
      the underlying file system may support such names, the Windows shell and
      user interface does not. However, it is acceptable to specify a period
      as the first character of a name. For example, ".temp".

---

Handy list to copy/paste:

<>:"/\|?*

---

Note: Other OSs and file systems may vary; but in general, the only forbidden characters
in filenames on Unix-like systems appear to be the forward slash (/) and the null byte.

Nice regex to find and replace invalid chars in file name.
[<>:"/\|?*]
example:
javascript:
"my file is * invalid ?.pdf".replace(/[<>:"/\|?*]/g,"");
php:
$fileName = preg_replace('/[<>:"/\|?*]/','','my file is * invalid ?.pdf');
c#
var fileName = (new Regex(@"[<>:""/\|?*]")).Replace("my file is * invalid ?.pdf","_");

Thanks for the contribution! Couple of notes:

\ is the escape character in most regex engines, so you'll need to repeat it to make sure it gets included in the character class and doesn't just escape the | after it: [<>:"/\\|?*] / "my file is \\ invalid ?.pdf".replace(/[<>:"/\\|?*]/g, "_");

Also, I'm not super confident in my PHP knowledge, but I think you'll need to double-escape the backslash: once because PHP treats it as an escape character in the string literal (even when using single quotes), and a second time for the regex engine. So I think you'll need a total of four \ characters: '/[<>:"/\\\\|?*]/' (gross).

(It looks like C# uses the @ prefix to denote verbatim strings, which look like Python's raw strings, and should only need a single escape for the regex engine. JS does not (yet) seem to offer unescaped string literals, but RegExp literals don't apply the additional layer of escaping.)

The URL is old. The new docs URL is now https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file

Thanks! I updated the URL 👍

Thanks, dude!

its very nice ,sir

JavaScript:

let toFilename=string=>string.replace(/\n/g," ").replace(/[<>:"/\\|?*\x00-\x1F]| +$/g,"").replace(/^(CON|PRN|AUX|NUL|COM[1-9]|LPT[1-9])$/,x=>x+"_");

Microsoft's documentation neglects to mention COM0 and LPT0 which explorer.exe has trouble with (even on Windows 10 20H2), possibly because of a bug.

function filenameValidator(fname, { replacement = "�" } = {}) {
    // https://stackoverflow.com/a/31976060
    // https://gist.github.com/doctaphred/d01d05291546186941e1b7ddc02034d3

    const fname_original = fname;

    // resolve multi-line, whitespace trimming
    fname = fname.split(/[\r\n]/).map(s => s.trim()).filter(s => s.length).join("  ");
    
    // forbidden characters
    // (after multi-line, because new-line-chars are themselves forbidden characters)
    fname = fname.replaceAll(/[<>:"\/\\\|?*\x00-\x1F]/g, replacement);

    // advanced trim
    fname = fname.replace(/\.$/, "");

    // empty filename
    if (!fname.length) {
        fname = '_';
    }
    
    // forbidden filenames
    if (fname.match(/^(CON|PRN|AUX|NUL|COM1|COM2|COM3|COM4|COM5|COM6|COM7|COM8|COM9|LPT1|LPT2|LPT3|LPT4|LPT5|LPT6|LPT7|LPT8|LPT9)(\..+)?$/)) {
        fname = `_${fname}`;
    }
    
    return {
        fname,
        isOriginal: (fname === fname_original),
    };
}

For .Net, there is https://docs.microsoft.com/en-us/dotnet/api/system.io.path.getinvalidpathchars?view=net-5.0: name?.IndexOfAny(Path.GetInvalidFileNameChars()) == -1 or name?.IndexOfAny(Path.GetInvalidPathChars()) == -1 depending if you want to validate a file name or a full path.

It will still not catch issues with file names like NUL or COM on FAT or NTFS.

For Python3:
import re
re.sub(r'[<>:/\|?*"]+',"",filename)

Can a filename have space under windows?

Yes it can, you can verify by creating one yourself

Python3 :

def cleanFilename(sourcestring,  removestring ="\"|%:/,.\\[]<>*?") :
    return ''.join([c for c in sourcestring if c not in removestring])

If you know of a good GitHub Action that detect (or replace) those filenames automatically, I'm looking for one.

I'd like to rename a filename with a "-" (hyphen) suffix but windows (7) refuses.
Do you guys think this is forbidden only at the end of filenames?

I'm struggling with "Any other character that the target file system does not allow." Is there a table of reserved characters for NTFS and the various FAT systems?