How to find out if string has already been URL encoded?

Question

How could I check if string has already been encoded?

For example, if I encode TEST==, I get TEST%3D%3D. If I again encode last string, I

Answer 1

Check your URL for suspicious characters[1]. List of candidates:

WHITE_SPACE ,", < , > , { , } , | , \ , ^ , ~ , [ , ] , . and `

I use:

private static boolean isAlreadyEncoded(String passedUrl) {
        boolean isEncoded = true;
        if (passedUrl.matches(".*[\\ \"\\<\\>\\{\\}|\\\\^~\\[\\]].*")) {
                isEncoded = false;
        }
        return isEncoded;
}

For the actual encoding I proceed with:

https://stackoverflow.com/a/49796882/1485527

Note: Even if your URL doesn't contain unsafe characters you might want to apply, e.g. Punnycode encoding to the host name. So there is still much space for additional checks.

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[1] A list of candidates can be found in the section "unsafe" of the URL spec at Page 2. In my understanding '%' or '#' should be left out in the encoding check, since these characters can occur in encoded URLs as well.