version used: 2.051
An matching optional capture works:

auto ms = match("ab", "(a(.*))?(b)");
assert(ms.captures.length == 4); // Ok
assert(ms.captures[0] == "ab"); // Ok
assert(ms.captures[1] == "a"); // Ok
assert(ms.captures[2] == ""); // Ok
assert(ms.captures[3] == "b"); // Ok


But if optional capture doesn't match :

auto ms = match("b", "(a(.*))?(b)"); // same issue with pattern "(a(.*))*(b)"
assert(ms.captures.length == 4); // Failed length = 1
assert(ms.captures[0] == "b"); // Ok
assert(ms.captures[1] == ""); // core.exception.AssertError@regex.d(1724): 1
assert(ms.captures[2] == ""); // core.exception.AssertError@regex.d(1724): 1
assert(ms.captures[3] == "b"); // core.exception.AssertError@regex.d(1724): 1

In Captures.length (line 1713 in v2.051):
        @property size_t length()
        {
            foreach (i; 0 .. matches.length)
            {
                if (matches[i].startIdx >= input.length) return i;
            }
            return matches.length;
        }

for matches[1] and matches[2], startIdx == endIdx == startIdx.max
but matches[3] is fine: startIdx == 0 and endIdx == 1

in RegexMatch.trymatch (line 2397 in v2.051) startIdx and endIdx are set only if matching:
            case engine.REparen:
                // ... pass
                if (!trymatch(pop, pop + len))
                    goto Lnomatch;
                pmatch[n + 1].startIdx = ss;
                pmatch[n + 1].endIdx = src;
                pc = pop + len;
                break;

and in RegexMatch.test (line 1905 in v2.051) startIdx and endIdx are initialized to max for each match:

            foreach (i; 0 .. engine.re_nsub + 1)
            {
                pmatch[i].startIdx = -1;
                pmatch[i].endIdx = -1;
            }


in RegexMatch.test (line 1905 in v2.051) initializing startIdx and endIdx to startindex instead of max seems to fix the problem:

            foreach (i; 0 .. engine.re_nsub + 1)
            {
                pmatch[i].startIdx = startindex;
                pmatch[i].endIdx = startindex;
            }