The possible region modes are actually composed of two orthogonal controls: whether or not anchors match at the boundaries of the region, and whether or not the boundaries are transparent to contructs that "look around" the input to see if a match is appropriate at a certain location.

Currently two of the four possibilities of these controls settings are offered but this is an incomplete solution. The two orthogonal concepts are artificially tied together by the API. The way to make the regions the most useful is to allow access to these controls independently of one another.

It is important that the default behavior be the most straightforward since we do not want to force new users to master the complexities of these controls if they do not need to. The settings of "strong regions" should be the behavior present by default, with anchors matching at the region boundaries and with opaque boundaries for the lookahead/behind, and boundary matchers.