Filename Restrictions and Sanitizing Regexs

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If the filename contains some invalid Unicode characters, or matches a reserved keyword in NTFS, then chrome.downloads.download will throw Error: Invalid filename.

1. At any position (start/middle/end), the following Unicode characters are invalid:

   • Control characters \p{Cc} (\u{0} is allowed at the middle)
   • : ? " * < > | ~ (NTFS reserved characters)
   • \ and / (NTFS & Chrome treat them as path separators instead of a character in filename, so Invalid filename error will NOT occur)
   • Format characters \p{Cf}
   • Non-characters \p{Cn}

   Zero-width joiner \u{200D} is commonly used to composite emojis and form a new emoji. However, this character is invalid as well, as its category is “Format characters”.

   The regex is /[:?"*<>|~/\\\u{1}-\u{1f}\u{7f}\u{80}-\u{9f}\p{Cf}\p{Cn}]/gu

2. At the start/end of filename, the following Unicode characters are invalid:

   • NUL character \u{0}
   • Line separator \p{Zl}
   • Paragraph separator \p{Zp}
   • Space separators \p{Zs}
   • A dot . (rule by NTFS)

   The regex is /^[.\u{0}\p{Zl}\p{Zp}\p{Zs}]|[.\u{0}\p{Zl}\p{Zp}\p{Zs}]$/gu

3. Reserved keywords in NTFS: Filenames of CON, PRN, AUX, NUL, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, LPT9 (case-insensitive), with or without file extension, are all invalid.

   The regex is /^(CON|PRN|AUX|NUL|COM[1-9]|LPT[1-9])(?=\.|$)/gui

Other categories of Unicode characters, including private-use \p{Co} and surrogate pairs \p{Cs}, are allowed at any position.

Final regex

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The final regex (for JavaScript) replacing all invalid characters with _:

var pattern = /[:?"*<>|~/\\\u{1}-\u{1f}\u{7f}\u{80}-\u{9f}\p{Cf}\p{Cn}]|^[.\u{0}\p{Zl}\p{Zp}\p{Zs}]|[.\u{0}\p{Zl}\p{Zp}\p{Zs}]$|^(CON|PRN|AUX|NUL|COM[1-9]|LPT[1-9])(?=\.|$)/gui;
filename.replaceAll(pattern, '_');

Note: These characters/filenames are also invalid in NTFS, but sometimes NTFS just deletes the character instead of displaying an error.

Determining the Invalid Unicode characters

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I wrote a script to try each Unicode character, and ran it on Google Chrome 126.0.6478.116 on Windows 11 23H2 22631.3737.

What the script does:

1. Call serviceWorker.postMessage('') in SW console to start/stop the for loop
2. Create an empty blob and an Object URL to the blob in offscreen document
3. Call chrome.downloads.download, pass the Unicode character as filename and the URL created
4. Try to catch Error: Invalid filename. The character is invalid if error caught, otherwise valid.
5. Store the result in chrome.storage.local
6. Clear the download history periodically to maintain performance

The invalid Unicode characters at the start/end are found. Within the set, some characters are invalid at the middle of a filename as well. So we run the script again, replacing filename: char with

filename: `0${char}0`

background.js:

var creating;

async function setup_offscreen_document() {
    const offscreenUrl = chrome.runtime.getURL("offscreen.html");
    const existingContexts = await chrome.runtime.getContexts({
        contextTypes: ['OFFSCREEN_DOCUMENT'],
        documentUrls: [offscreenUrl]
    });

    if (existingContexts.length > 0) {
        return;
    }

    if (creating) {
        await creating;
    } else {
        creating = chrome.offscreen.createDocument({
            url: "offscreen.html",
            reasons: ['BLOBS'],
            justification: 'Create object URLs',
        });
        await creating;
        creating = null;
    }
}

var stop = true;
var url;

async function test(code) {
    var char = String.fromCodePoint(code);
    try {
        chrome.downloads.cancel(await chrome.downloads.download({ url: url, filename: char }));
        chrome.storage.local.set({ [code]: { char: char, invalid: false } });
    } catch (error) {
        if (error.message == "Invalid filename")
            chrome.storage.local.set({ [code]: { char: char, invalid: true } });
        else
            throw error;
    }
}

async function loop() {
    await setup_offscreen_document();
    url = await chrome.runtime.sendMessage(undefined, {});
    var start = (await chrome.storage.local.get('last'))['last'];
    if (start == undefined)
        start = 0;
    for (var i = start + 1; i <= 0x10ffff; i++) {
        await test(i);
        await chrome.storage.local.set({ last: i });
        if ((i - start) % 10000 == 0)
            chrome.browsingData.removeDownloads({});
        if (stop)
            break;
    }
}

function start_loop() {
    stop = false;
    loop();
}

function stop_loop() {
    stop = true;
}

self.addEventListener('message', function (e) {
    if (stop)
        start_loop();
    else
        stop_loop();
});

manifest.json:

{
    "manifest_version": 3,
    "name": "Unicode Test",
    "version": "1.0",
    "description": "Test.",
    "background": {
        "service_worker": "background.js",
        "type": "module"
    },
    "permissions": [
        "downloads",
        "storage",
        "unlimitedStorage",
        "offscreen",
        "browsingData"
    ]
}