想到
my other problem，我决定我甚至不能创建一个符合罗马数字的正则表达式(更不用说上下文无关的语法，将生成它们)
问题是只匹配有效的罗马数字。
例如，990不是“XM”，它的“CMXC”
我的问题在制作正则表达式，这是为了允许或不允许某些字符，我需要回头看。
让我们以数千为例。
我可以允许M {0,2} C？M(允许900,1000,1900,2000,2900和3000)。但是，如果匹配在CM上，我不能允许以下字符是C或D(因为我已经在900)。
如何在正则表达式中表达？
如果它在正则表达式中根本不可表达，那么它在上下文无关语法中是否可表达？
尝试：
原文链接：https://www.f2er.com/regex/358294.html^M{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$
打破它：
M{0,4}
这指定了数千部分，基本上限制在0和4000之间。这是一个相对简单：
0: <empty> matched by M{0}
1000: M matched by M{1}
2000: MM matched by M{2}
3000: MMM matched by M{3}
4000: MMMM matched by M{4}
(CM|CD|D?C{0,3})
稍微复杂一点，这是为百个部分，涵盖所有的可能性：
0: <empty> matched by D?C{0} (with D not there)
100: C matched by D?C{1} (with D not there)
200: CC matched by D?C{2} (with D not there)
300: CCC matched by D?C{3} (with D not there)
400: CD matched by CD
500: D matched by D?C{0} (with D there)
600: DC matched by D?C{1} (with D there)
700: DCC matched by D?C{2} (with D there)
800: DCCC matched by D?C{3} (with D there)
900: CM matched by CM
(XC|XL|L?X{0,3})
与上一节相同的规则，但十位：
0: <empty> matched by L?X{0} (with L not there)
10: X matched by L?X{1} (with L not there)
20: XX matched by L?X{2} (with L not there)
30: XXX matched by L?X{3} (with L not there)
40: XL matched by XL
50: L matched by L?X{0} (with L there)
60: LX matched by L?X{1} (with L there)
70: LXX matched by L?X{2} (with L there)
80: LXXX matched by L?X{3} (with L there)
90: XC matched by XC
(IX|IV|V?I{0,3})
这是单位部分，处理0到9，也类似于前两个部分(罗马数字，尽管它们看起来奇怪，遵循一些逻辑规则，一旦你弄清楚它们是什么)：
0: <empty> matched by V?I{0} (with V not there)
1: I matched by V?I{1} (with V not there)
2: II matched by V?I{2} (with V not there)
3: III matched by V?I{3} (with V not there)
4: IV matched by IV
5: V matched by V?I{0} (with V there)
6: VI matched by V?I{1} (with V there)
7: VII matched by V?I{2} (with V there)
8: VIII matched by V?I{3} (with V there)
9: IX matched by IX
