findLastSplitAfter

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

auto f()() @safe pure nothrow { char[1] x = "_"; return x[].findLastSplitAfter(' '); }
static if (hasPreviewDIP1000) static assert(!__traits(compiles, { auto _ = f(); }));

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

auto r = "a*b*c".findLastSplitAfter('*');
static assert(is(typeof(r.pre()) == string));
static assert(is(typeof(r.post()) == string));
assert(r);
assert(r.pre == "a*b*");
assert(r.post == "c");