I stumbled over this:
---
import std.regex;
void main() {
std.regex.match("a"d, "aa"d);
}
---
=> core.exception.AssertError@/home/nils/d/dmd2/linux/bin32/../../src/phobos/std/utf.d(800): Attempted to decode past the end of a string

I traced it down to ShiftOr.search returning a value greater than the input length:
---
dstring
	input = "a",
	pattern = "aa";
assert(regex(pattern).kickstart.search(input, 0) <= input.length); // fails
---