Java/android - check if string `FileName` contains any file-name-disallowed characters

Question

I am checking if a string filename is safe for saving by checking if it has any strange characters with a Regular expression. It is will be a .json file and should also allow spaces in the file name.

public static boolean fileNameIsSafe(String nameIncExt) {
        Matcher m = Pattern.compile("regex?").matcher(nameIncExt);
        return m.matches();
    }

What would the regular expression be?

Answer 1

There is probably a more elegant solution - I'm not a regex expert, but how about this:

[.]*\\w[\\w| |.]*

Breaking it down:

[.]* 

The filename can start with 0 or more '.'s

\w  

Then you need at least one word character, which can be az, AZ, 0-9, or _. this prevents file names of '...' or file names that start with a space, or only contain '.'s and spaces.

[\w| |.]*

The remaining file name can have 0 or more word characters, spaces, and/or '.' I'm assuming you can trim off trailing spaces.

Note in Java you'll need to escape the backslashes, so it would look like this:

boolean isSafeFilename = Pattern.matches("[.]*\\w[\\w| |.]*", stringToTest);

Answer 2

I get the feeling that you've never visited this site: Regular-Expressions

Anyway, you can match characters within a character class: [/\\]\\[;']