Because Perl returns a string with a newline at the end when reading a line from a file, Perl’s regex engine matches $ at the position before the line break at the end of the string even when multi-line mode is turned off. Perl also matches $ at the very end of the string, regardless of whether that character is a line break. So ^\d+$ matches 123 whether the subject string is 123 or 123\n.
Most modern regex flavors have copied this behavior. That includes .NET, Java, PCRE, Delphi, PHP, and Python. This behavior is independent of any settings such as “multi-line mode”.
In all these flavors except Python, \Z also matches before the final line break. If you only want a match at the absolute very end of the string, use \z (lowercase z instead of uppercase Z). \A\d+\z does not match 123\n. \z matches after the line break, which is not matched by the shorthand character class.
