title: Java - Roman Numeral validity
body: <p>I am writing a program to add two roman numerals without converting to any bases. I have everything working except I am not sure how to check if my input String is a valid roman numeral or not. </p>
  
  <p>These are the rules to check validity:</p>
  
  <ol>
  <li>Five in a row of any digit is not allowed</li>
  <li>Some digits are allowed in runs of up to 4. They are I,X,C, and M. The others (V,L,D) can only appear singly.</li>
  <li>Some lower digits can come before a higher digit, but only if they appear singly. E.g. "IX" is ok but "IIIX" is not.</li>
  <li>But this is only for pairs of digits. Three ascending numbers in a row is invalid. E.g. "IX" is ok but "IXC" is not.</li>
  <li>A single digit with no runs is always allowed</li>
  </ol>
  
  <p>I have not really been able to make much progress with this step and don't have anything working yet. Any help would be great!</p>