findSplitBefore

char[] haystack;
auto r = haystack.findSplitBefore('_');
static assert(is(typeof(r.pre()) == typeof(haystack)));
static assert(is(typeof(r.post()) == typeof(haystack)));
assert(!r);
assert(!r.pre);
assert(!r.post);
version (unittest) {
	static auto f()() @safe pure nothrow { char[1] x = "_"; return x[].findSplitBefore(' '); }
	static if (hasPreviewDIP1000) static assert(!__traits(compiles, { auto _ = f(); }));
}

const(char)[] haystack;
auto r = haystack.findSplitBefore('_');
static assert(is(typeof(r.pre()) == typeof(haystack)));
static assert(is(typeof(r.post()) == typeof(haystack)));
assert(!r);
assert(!r.pre);
assert(!r.post);

const r = "a*b".findSplitBefore('*');
assert(r);
assert(r.pre == "a");
assert(r.post == "*b");

const r = "a*b".findSplitBefore('_');
assert(!r);
assert(r.pre == "a*b");
assert(r.post == "");