PHP json_decode a json_encoded string - php

Question

I don't really get the following...
// array to encode
$a = ['regex' => '\/\+\/'];
// decoding an encoded array works
print_r(json_decode(json_encode($a), true));
// encoded array
echo json_encode($a);
// trying to decode the before encoded string doesn't work
print_r(json_decode('{"regex":"\\\/\\+\\\/"}', true));
echo json_last_error_msg();
The last error message says Syntax error. Shouldn't I be able to easily decode a simple encoded string?
I know that the problem is in the backslashes but I won't want to do any magic string replacement or regex to get the decoding working. Just want to understand where goes what wrong and what's a best practice for this kind of situations?
I'm using PHP version 5.5.9

Answer 1

Found something thanks to that answer : https://stackoverflow.com/a/10929508/1685538
If instead of taking the string outputted by echo, you use var_export(json_encode($a)); (documentation), it gives you {"regex":"\\\\\\/\\\\+\\\\\\/"}
print_r(json_decode('{"regex":"\\\\\\/\\\\+\\\\\\/"}', true)); gives the expected result :
Array
(
[regex] => \/\+\/
)
with no errors.

Answer 2

The problem is that using backslashes in PHP code string literals is subject to PHP's backslash escaping rules. You need to additionally escape the backslashes so they are preserved inside the PHP string:
print_r(json_decode('{"regex":"\\\\\\/\\\\+\\\\\\/"}', true));
Contrast with:
echo '{"regex":"\\\/\\+\\\/"}';
// {"regex":"\\/\+\\/"}