$ is a zero-width assertion that matches at the end of the string, or just before the newline at the end of the string (note its behavior can be changed by the /m modifier). ? makes the preceding .* non-greedy, so that it allows $ to match just before the newline, while a normal .* is greedy, causing it to gobble up everything it can, and the $ matches just after the newline. Note that since you're using the /s modifier, the dot . can also match a newline, which it normally does not.
What is your desired behavior? If you want the match to always include the newline, I'd be specific about it: /ab.*\n/, or, if you want the newline to be optional (e.g. the last line in a file may not have a newline), then I'd probably write /ab.*(?:\z|\n)/. If you want the match to always exclude the newline, I'd leave off the /s modifier, or be specific about what you want to match by saying e.g. [^\n]* or \N*.
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