void Parse()

in rapidjson/internal/regex.h [203:338]
113 lines of code
38 McCabe index (conditional complexity)

    void Parse(DecodedStream<InputStream>& ds) {
        Allocator allocator;
        Stack<Allocator> operandStack(&allocator, 256);     // Frag
        Stack<Allocator> operatorStack(&allocator, 256);    // Operator
        Stack<Allocator> atomCountStack(&allocator, 256);   // unsigned (Atom per parenthesis)

        *atomCountStack.template Push<unsigned>() = 0;

        unsigned codepoint;
        while (ds.Peek() != 0) {
            switch (codepoint = ds.Take()) {
                case '^':
                    anchorBegin_ = true;
                    break;

                case '$':
                    anchorEnd_ = true;
                    break;

                case '|':
                    while (!operatorStack.Empty() && *operatorStack.template Top<Operator>() < kAlternation)
                        if (!Eval(operandStack, *operatorStack.template Pop<Operator>(1)))
                            return;
                    *operatorStack.template Push<Operator>() = kAlternation;
                    *atomCountStack.template Top<unsigned>() = 0;
                    break;

                case '(':
                    *operatorStack.template Push<Operator>() = kLeftParenthesis;
                    *atomCountStack.template Push<unsigned>() = 0;
                    break;

                case ')':
                    while (!operatorStack.Empty() && *operatorStack.template Top<Operator>() != kLeftParenthesis)
                        if (!Eval(operandStack, *operatorStack.template Pop<Operator>(1)))
                            return;
                    if (operatorStack.Empty())
                        return;
                    operatorStack.template Pop<Operator>(1);
                    atomCountStack.template Pop<unsigned>(1);
                    ImplicitConcatenation(atomCountStack, operatorStack);
                    break;

                case '?':
                    if (!Eval(operandStack, kZeroOrOne))
                        return;
                    break;

                case '*':
                    if (!Eval(operandStack, kZeroOrMore))
                        return;
                    break;

                case '+':
                    if (!Eval(operandStack, kOneOrMore))
                        return;
                    break;

                case '{':
                    {
                        unsigned n, m;
                        if (!ParseUnsigned(ds, &n))
                            return;

                        if (ds.Peek() == ',') {
                            ds.Take();
                            if (ds.Peek() == '}')
                                m = kInfinityQuantifier;
                            else if (!ParseUnsigned(ds, &m) || m < n)
                                return;
                        }
                        else
                            m = n;

                        if (!EvalQuantifier(operandStack, n, m) || ds.Peek() != '}')
                            return;
                        ds.Take();
                    }
                    break;

                case '.':
                    PushOperand(operandStack, kAnyCharacterClass);
                    ImplicitConcatenation(atomCountStack, operatorStack);
                    break;

                case '[':
                    {
                        SizeType range;
                        if (!ParseRange(ds, &range))
                            return;
                        SizeType s = NewState(kRegexInvalidState, kRegexInvalidState, kRangeCharacterClass);
                        GetState(s).rangeStart = range;
                        *operandStack.template Push<Frag>() = Frag(s, s, s);
                    }
                    ImplicitConcatenation(atomCountStack, operatorStack);
                    break;

                case '\\': // Escape character
                    if (!CharacterEscape(ds, &codepoint))
                        return; // Unsupported escape character
                    // fall through to default

                default: // Pattern character
                    PushOperand(operandStack, codepoint);
                    ImplicitConcatenation(atomCountStack, operatorStack);
            }
        }

        while (!operatorStack.Empty())
            if (!Eval(operandStack, *operatorStack.template Pop<Operator>(1)))
                return;

        // Link the operand to matching state.
        if (operandStack.GetSize() == sizeof(Frag)) {
            Frag* e = operandStack.template Pop<Frag>(1);
            Patch(e->out, NewState(kRegexInvalidState, kRegexInvalidState, 0));
            root_ = e->start;

#if RAPIDJSON_REGEX_VERBOSE
            printf("root: %d\n", root_);
            for (SizeType i = 0; i < stateCount_ ; i++) {
                State& s = GetState(i);
                printf("[%2d] out: %2d out1: %2d c: '%c'\n", i, s.out, s.out1, (char)s.codepoint);
            }
            printf("\n");
#endif
        }

        // Preallocate buffer for SearchWithAnchoring()
        RAPIDJSON_ASSERT(stateSet_ == 0);
        if (stateCount_ > 0) {
            stateSet_ = static_cast<unsigned*>(states_.GetAllocator().Malloc(GetStateSetSize()));
            state0_.template Reserve<SizeType>(stateCount_);
            state1_.template Reserve<SizeType>(stateCount_);
        }
    }