There is a missing regex delimiter (the "/" character) causing error when calling preg_match(). This bug has existed at least since 1.4.20 or earlier. I usually patched this on our private installation.
There is no bug here. The regular expression being used in this assignment is built in functions/url_parser.php, where the closing slash is already added. If you remove your change and still get preg_match() errors, please show those errors.
The error message:
Warning: preg_match_all() [function.preg-match-all]: No ending delimiter '/' found in /home/sloki/spanel/webapps/webmail/functions/mime.php on line 2356
This used to happen in 1.4.13 when opening some file attachments. Since then I have patched our installation whenever upgrading (on 1.4.19, 1.4.20, and lately 1.4.21). Sorry if this has been fixed in between 1.4.13 and later versions?
That error shows a line number that is not applicable to any recent version of SquirrelMail. As I said, please un-do your change and try to reproduce the error and send it if you get one. Also please specify what actions you need to take to get the error. You can also look in the code yourself to find where the pattern is built (I've already told you where it is).
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