Easiest Way to Check If an Arbitrary String Is a Valid Filename

Easiest way to check if an arbitrary String is a valid filename

Check whether filename.IndexOfAny(Path.GetInvalidFileNameChars()) < 0 and !File.Exists(Path.Combine(someFolder, filename))

How do I check if a given string is a legal/valid file name under Windows?

You can get a list of invalid characters from Path.GetInvalidPathChars and GetInvalidFileNameChars.

UPD: See Steve Cooper's suggestion on how to use these in a regular expression.

UPD2: Note that according to the Remarks section in MSDN "The array returned from this method is not guaranteed to contain the complete set of characters that are invalid in file and directory names." The answer provided by sixlettervaliables goes into more details.

Check if string is valid filename

You may have a look in php's basename function, it will return with filename, see example below:

$file = '../../abc.txt';
echo basename($file); //output: abc.txt

Note: basename gets you the file name from path string irrespective of file physically exists or not. file_exists function can be used to verify that the file physically exists.

How to make a valid Windows filename from an arbitrary string?

Try something like this:

string fileName = "something";
foreach (char c in System.IO.Path.GetInvalidFileNameChars())
{
   fileName = fileName.Replace(c, '_');
}

Edit:

Since GetInvalidFileNameChars() will return 10 or 15 chars, it's better to use a StringBuilder instead of a simple string; the original version will take longer and consume more memory.

How to make a valid filename from an arbitrary string in Javascript?

Huge thanks to Kelvin's answer!

I quickly compiled it into a function. The final code I used is:

function convertToValidFilename(string) {
    return (string.replace(/[\/|\\:*?"<>]/g, " "));
}

var string = 'Un éléphant à l\'orée du bois/An elephant at the edge of the woods".txt';

console.log("Before = ", string);
console.log("After  = ", convertToValidFilename(string));

This results in the output:

Before =  Un éléphant à l'orée du bois/An elephant at the edge of the woods".txt
After  =  Un éléphant à l orée du bois An elephant at the edge of the woods .txt

Turn a string into a valid filename?

You can look at the Django framework for how they create a "slug" from arbitrary text. A slug is URL- and filename- friendly.

The Django text utils define a function, slugify(), that's probably the gold standard for this kind of thing. Essentially, their code is the following.

import unicodedata
import re

def slugify(value, allow_unicode=False):
    """
    Taken from https://github.com/django/django/blob/master/django/utils/text.py
    Convert to ASCII if 'allow_unicode' is False. Convert spaces or repeated
    dashes to single dashes. Remove characters that aren't alphanumerics,
    underscores, or hyphens. Convert to lowercase. Also strip leading and
    trailing whitespace, dashes, and underscores.
    """
    value = str(value)
    if allow_unicode:
        value = unicodedata.normalize('NFKC', value)
    else:
        value = unicodedata.normalize('NFKD', value).encode('ascii', 'ignore').decode('ascii')
    value = re.sub(r'[^\w\s-]', '', value.lower())
    return re.sub(r'[-\s]+', '-', value).strip('-_')

And the older version:

def slugify(value):
    """
    Normalizes string, converts to lowercase, removes non-alpha characters,
    and converts spaces to hyphens.
    """
    import unicodedata
    value = unicodedata.normalize('NFKD', value).encode('ascii', 'ignore')
    value = unicode(re.sub('[^\w\s-]', '', value).strip().lower())
    value = unicode(re.sub('[-\s]+', '-', value))
    # ...
    return value

There's more, but I left it out, since it doesn't address slugification, but escaping.

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