Regex doesn't match the string in Google Analytics

wish of those help I though both regex should work but I got same problem as you in regex101. So, in order to fix this you can change your regex to:

$row = "TEDARİKÇİ,MÜŞTERİ";
var_dump( preg_match('#\bMÜŞTERİ(\b|$)#iu', $row));

With these it helps Forgive me if Google Analytics has some regex standards which I am overlooking but is it possible that your regex is failing because it does not account for the start of the whole of the URL? Adding .* to either end of your regex may help.
It also looks like your regex is over-complex for the conditions you have described. Could a simpler match be :

may help you . My assumption is your goal Destination is not set to Regular expression.
If you want to use regex in your steps, you have to specify Regular expression in Destination

Any of those help I'm trying to find a Regex that matches a word but doesn't start/end with a specific character. , You can use lookaround based regex in PHP (PCRE):

(?<![a-zA-Z0-9])mini(?![a-zA-Z0-9])

Does that help Some observations:
/* looks dubious as a regular expression for Index. It will match a slash character any number of times, including zero times. In other words, it will also match nothing - see demo. As others have pointed out, to match a literal question mark, it needs to be escaped by preceding with a backslash - so \? instead of ?. As far as I can tell, Availability(\?.*|$) ought to work - see demo. Are you sure the problem isn't elsewhere? The final three regular expressions may or may not be sufficient - if there might be the possibility of something like /Booking/Indexer for example, the /Booking/Index.* regular expression would match this, which could be wrong - see demo. To rule this out, consider an alternative such as /Booking/Index([/?].*|$) - see demo.