What does %5B and %5D in POST requests stand for?

As per this answer over here: str="foo%20%5B12%5D" encodes foo [12]:

%20 is space
%22 is quotes
%5B is '['
and %5D is ']'

This is called percent encoding and is used in encoding special characters in the url parameter values.

EDIT By the way as I was reading https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/encodeURI#Description, it just occurred to me why so many people make the same search. See the note on the bottom of the page:

Also note that if one wishes to follow the more recent RFC3986
for URL’s, making square brackets reserved (for IPv6) and thus not
encoded when forming something which could be part of a URL (such as a
host), the following may help.

function fixedEncodeURI (str) {
    return encodeURI(str).replace(/%5B/g, '[').replace(/%5D/g, ']');
}

Hopefully this will help people sort out their problems when they stumble upon this question.

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