REG_NEWLINE Compile for newline-sensitive matching. By default, newline is a completely ordinary character with no special meaning in either REs or strings. With this flag, `[^' bracket expressions and `.' never match newline, a `^' anchor matches the null string after any newline in the string in addition to its normal function, and the `$' anchor matches the null string before any newline in the string in addition to its normal function.
Using ^ and $ to match near newlines makes matching patterns that cross lines much easier to control. But anchors are also a source of bugs for some implementations, e.g. libtre.
Maybe I underestimated the utility of ^ and $. The definition seems intricate. I thought about adding a combinator for matching newline but now think that would lead to wrong start and end positions. For example the start position of the matching substring for ^a in "a \na" should be 2 not 1, right? Or is it 0 although there is no newline at the beginning? Is there a page with examples that show how ^ and $ should behave exactly? Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)