System.Text.RegularExpressions.RegexParser.ScanBasicBackslash C# (CSharp) Method

        internal RegexNode ScanBasicBackslash()
        {
            if (CharsRight() == 0)
                throw MakeException(SR.IllegalEndEscape);

            char ch;
            bool angled = false;
            char close = '\0';
            int backpos;

            backpos = Textpos();
            ch = RightChar();

            // allow \k<foo> instead of \<foo>, which is now deprecated

            if (ch == 'k')
            {
                if (CharsRight() >= 2)
                {
                    MoveRight();
                    ch = MoveRightGetChar();

                    if (ch == '<' || ch == '\'')
                    {
                        angled = true;
                        close = (ch == '\'') ? '\'' : '>';
                    }
                }

                if (!angled || CharsRight() <= 0)
                    throw MakeException(SR.MalformedNameRef);

                ch = RightChar();
            }

            // Note angle without \g

            else if ((ch == '<' || ch == '\'') && CharsRight() > 1)
            {
                angled = true;
                close = (ch == '\'') ? '\'' : '>';

                MoveRight();
                ch = RightChar();
            }

            // Try to parse backreference: \<1> or \<cap>

            if (angled && ch >= '0' && ch <= '9')
            {
                int capnum = ScanDecimal();

                if (CharsRight() > 0 && MoveRightGetChar() == close)
                {
                    if (IsCaptureSlot(capnum))
                        return new RegexNode(RegexNode.Ref, _options, capnum);
                    else
                        throw MakeException(SR.Format(SR.UndefinedBackref, capnum.ToString(CultureInfo.CurrentCulture)));
                }
            }

            // Try to parse backreference or octal: \1

            else if (!angled && ch >= '1' && ch <= '9')
            {
                if (UseOptionE())
                {
                    int capnum = -1;
                    int newcapnum = (int)(ch - '0');
                    int pos = Textpos() - 1;
                    while (newcapnum <= _captop)
                    {
                        if (IsCaptureSlot(newcapnum) && (_caps == null || (int)_caps[newcapnum] < pos))
                            capnum = newcapnum;
                        MoveRight();
                        if (CharsRight() == 0 || (ch = RightChar()) < '0' || ch > '9')
                            break;
                        newcapnum = newcapnum * 10 + (int)(ch - '0');
                    }
                    if (capnum >= 0)
                        return new RegexNode(RegexNode.Ref, _options, capnum);
                }
                else
                {
                    int capnum = ScanDecimal();
                    if (IsCaptureSlot(capnum))
                        return new RegexNode(RegexNode.Ref, _options, capnum);
                    else if (capnum <= 9)
                        throw MakeException(SR.Format(SR.UndefinedBackref, capnum.ToString(CultureInfo.CurrentCulture)));
                }
            }

            else if (angled && RegexCharClass.IsWordChar(ch))
            {
                string capname = ScanCapname();

                if (CharsRight() > 0 && MoveRightGetChar() == close)
                {
                    if (IsCaptureName(capname))
                        return new RegexNode(RegexNode.Ref, _options, CaptureSlotFromName(capname));
                    else
                        throw MakeException(SR.Format(SR.UndefinedNameRef, capname));
                }
            }

            // Not backreference: must be char code

            Textto(backpos);
            ch = ScanCharEscape();

            if (UseOptionI())
                ch = _culture.TextInfo.ToLower(ch);

            return new RegexNode(RegexNode.One, _options, ch);
        }