The results did not surprise me. The fact that everyone is in agreement that "cat$" matches "cat" and not "cat\n" if multiline is off did not surprise me. \n is

The results did not surprise me. The fact that everyone is in agreement that "cat$" matches "cat" and not "cat\n" if multiline is off did not surprise me. \n is implicitly a multiline-contextual character to me. In other words, if you didn't have any \n, you'd just have an array of lines (without linefeeds), same as if you were reading lines from a file one at a time or splitting a binary on \n. The other results that differ across engines seem to be because people either don't understand regex or because the POSIX description of how to deal with such an input and config was ill-defined.