The solution (which other answers don't mention—at least at the time of my originally writing this) is that when PHP refers to delimiters, it's not referring to the delimiters you see in your code (which are quote marks) but the next characters inside the string. (In fact I've never seen this stated anywhere in any documentation: you have to see it in examples.) So instead of having a regular expression syntax like what you may be accustomed to from many other languages:
/something/
PHP uses strings, and then looks inside the string for another delimiter:
'/something/'
The delimiter PHP is referring to is the pair of / characters, instead of the pair of ' characters. So if you write 'something', PHP will take s as the intended delimiter and complain that you're not allowed to use alphanumeric characters as your delimiter.
So if you want to pass (for instance) an i to show that you want a case-insensitve match, you pass it inside the string but outside of the regex delimiters:
'/something/i'
If you want to use something other than / as your delimiter, you can, such as if you're matching a URL and don't want to have to escape all the slashes:
'~something~'