Try this pattern:
^[^IVXMCDL]*$
This will match the start of the string, followed by zero or more characters other than those specified in the character class, followed by the end of the string. In other words, it will not match any a string which contains those characters.
Also note that depending on how you're using it, you could probably use a simpler pattern like this:
[IVXMCDL]
And reject any string which matches the pattern.
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