System.Xml.Xsl.XsltOld.Compiler.getXPathLex C# (CSharp) Method

        private static void getXPathLex(string avt, ref int start, StringBuilder lex)
        {
            // AVT parser states
            const int InExp = 0;      // Inside AVT expression
            const int InLiteralA = 1;      // Inside literal in expression, apostrophe delimited
            const int InLiteralQ = 2;      // Inside literal in expression, quote delimited
            Debug.Assert(avt.Length != start, "Empty string not supposed here");
            Debug.Assert(lex.Length == 0, "Builder shoud to be reset here");
            Debug.Assert(avt[start] == '{', "We calling getXPathLex() only after we realy meet {");
            int avtLength = avt.Length;
            int state = InExp;
            for (int i = start + 1; i < avtLength; i++)
            {
                char ch = avt[i];
                switch (state)
                {
                    case InExp:
                        switch (ch)
                        {
                            case '{':
                                throw XsltException.Create(SR.Xslt_NestedAvt, avt);
                            case '}':
                                i++; // include '}'
                                if (i == start + 2)
                                { // empty XPathExpresion
                                    throw XsltException.Create(SR.Xslt_EmptyAvtExpr, avt);
                                }
                                lex.Append(avt, start + 1, i - start - 2); // avt without {}
                                start = i;
                                return;
                            case '\'':
                                state = InLiteralA;
                                break;
                            case '"':
                                state = InLiteralQ;
                                break;
                        }
                        break;
                    case InLiteralA:
                        if (ch == '\'')
                        {
                            state = InExp;
                        }
                        break;
                    case InLiteralQ:
                        if (ch == '"')
                        {
                            state = InExp;
                        }
                        break;
                }
            }

            // if we meet end of string before } we have an error
            throw XsltException.Create(state == InExp ? SR.Xslt_OpenBracesAvt : SR.Xslt_OpenLiteralAvt, avt);
        }