What characters are forbidden in Windows and Linux directory names?

Question

I know that / is illegal in Linux, and the following are illegal in Windows (I think) * . \" / \\ [

Answer 1

Here's a c# implementation for windows based on Christopher Oezbek's answer

It was made more complex by the containsFolder boolean, but hopefully covers everything

/// 

/// This will replace invalid chars with underscores, there are also some reserved words that it adds underscore to
/// 
/// 
/// https://stackoverflow.com/questions/1976007/what-characters-are-forbidden-in-windows-and-linux-directory-names
/// 
/// Pass in true if filename represents a folder\file (passing true will allow slash)
public static string EscapeFilename_Windows(string filename, bool containsFolder = false)
{
    StringBuilder builder = new StringBuilder(filename.Length + 12);

    int index = 0;

    // Allow colon if it's part of the drive letter
    if (containsFolder)
    {
        Match match = Regex.Match(filename, @"^\s*[A-Z]:\\", RegexOptions.IgnoreCase);
        if (match.Success)
        {
            builder.Append(match.Value);
            index = match.Length;
        }
    }

    // Character substitutions
    for (int cntr = index; cntr < filename.Length; cntr++)
    {
        char c = filename[cntr];

        switch (c)
        {
            case '\u0000':
            case '\u0001':
            case '\u0002':
            case '\u0003':
            case '\u0004':
            case '\u0005':
            case '\u0006':
            case '\u0007':
            case '\u0008':
            case '\u0009':
            case '\u000A':
            case '\u000B':
            case '\u000C':
            case '\u000D':
            case '\u000E':
            case '\u000F':
            case '\u0010':
            case '\u0011':
            case '\u0012':
            case '\u0013':
            case '\u0014':
            case '\u0015':
            case '\u0016':
            case '\u0017':
            case '\u0018':
            case '\u0019':
            case '\u001A':
            case '\u001B':
            case '\u001C':
            case '\u001D':
            case '\u001E':
            case '\u001F':

            case '<':
            case '>':
            case ':':
            case '"':
            case '/':
            case '|':
            case '?':
            case '*':
                builder.Append('_');
                break;

            case '\\':
                builder.Append(containsFolder ? c : '_');
                break;

            default:
                builder.Append(c);
                break;
        }
    }

    string built = builder.ToString();

    if (built == "")
    {
        return "_";
    }

    if (built.EndsWith(" ") || built.EndsWith("."))
    {
        built = built.Substring(0, built.Length - 1) + "_";
    }

    // These are reserved names, in either the folder or file name, but they are fine if following a dot
    // CON, PRN, AUX, NUL, COM0 .. COM9, LPT0 .. LPT9
    builder = new StringBuilder(built.Length + 12);
    index = 0;
    foreach (Match match in Regex.Matches(built, @"(^|\\)\s*(?CON|PRN|AUX|NUL|COM\d|LPT\d)\s*(\.|\\|$)", RegexOptions.IgnoreCase))
    {
        Group group = match.Groups["bad"];
        if (group.Index > index)
        {
            builder.Append(built.Substring(index, match.Index - index + 1));
        }

        builder.Append(group.Value);
        builder.Append("_");        // putting an underscore after this keyword is enough to make it acceptable

        index = group.Index + group.Length;
    }

    if (index == 0)
    {
        return built;
    }

    if (index < built.Length - 1)
    {
        builder.Append(built.Substring(index));
    }

    return builder.ToString();
}