Las URI segment is zero (/0) -> error DIsallowed characters - codeigniter

Question

I tried to find a solution for this issue but nothing worked. When my REST api URI request is, ex. https://serverip/meeting/userlist/0
I always get the error "The URI you submitted has disallowed characters”. I have even tried to leave this parameter in the config file blank:
$config['permitted_uri_chars'] = 'a-z 0-9~%.:_-+';
But I get the same error.
Is not allowed to have a 0 at the end of the URI as unique content of that segment? Because I need that to retrieve user with id = 0.
Thanks a lot.
EDIT - SOLVED:
Hi Again,
finally I solved it. I found that long time ago we commented a check related to UTF8 encodig in URI.php
if ( ! empty($str) && ! empty($this->_permitted_uri_chars) && ! preg_match('/^['.$this->_permitted_uri_chars.']+$/i'.(UTF8_ENABLED ? 'u' : ''), $str))
And we only left the first condition. We had some code issues that seem not to reproduce after revert that comment. And /0 now works fine.
So sorry, at the end it was a problem related to our own modifications.
Thanks.

Answer 1

$config['permitted_uri_chars'] is used as a PCRE character class pattern.
With the last character in there being a dash, it looks for a dash. However, when a dash is between two characters, it triggers a range search. So ... when you append the + (plus) sign after the dash, you get:
[_-+] // a range between underscore and plus in the ASCII table
You might be thinking "So what? Zeros are already allowed previously via 0-9", and you'd be correct, but that's not the problem. The problem is that the plus sign has a lower ASCII number than the underscore, and ranges don't work backwards, so _-+ is invalid and triggers a PCRE compilation failure, which in turn means the entire check fails and nothing is actually allowed.
You would see this if you had error_reporting enabled and/or looked at the error logs.
This doesn't happen if you only append the plus sign to the default pattern - the dash is not only the last character, but also escaped with a backslash - as you'd have this instead:
[_\-+] // Underscore, dash and plus sign as individual characters; not a range
I guess you thought it was an actual character to be allowed and removed it. Just add it back:
$config['permitted_uri_chars'] = 'a-z 0-9~%.:_\-+';