How to create regular expression checking Roman numerals?

Question

I need to create regular expression which verifies if user inputs:

• 4 digits OR
• value like XXXXXX-YY, where X is roman numerals from I to XXXIII and YY is two latin characters (A-Z)

Answer 1

According to the requirements, these are possible roman number-formats. For readability, only the maximum number of X is shown.

XXX III     (or: <empty>, I or II instead of III)
XX V       (or: IV, IX and X instead of IV)

I suggest this compact pattern:

/^(\d{4}|(?=[IVX])(X{0,3}I{0,3}|X{0,2}VI{0,3}|X{0,2}I?[VX])-[A-Z]{2})$/i

Explanation:

^                Begin of string
(                Begin of group 1.
  \d{4}             4 digits

|                 OR

  (?=[IVX])         Look-ahead: Must be followed by a I, V or X
  (                  Begin of group 2.
     X{0,3}I{0,3}       = 0 1 2 3  + { 0 ; 10 ; 20 ; 30} (roman)
  |                  OR
     X{0,2}VI{0,3}      = 5 6 7 8  + { 0 ; 10 ; 20 }     (roman)
  |                  OR
     X{0,2}I?[VX]       = 4 9      + { 0 ; 10 ; 20 }     (roman)
  )                  End of group 2
  -[A-Z]{2}          Postfixed by a hyphen and two letters
)                 End of group 1.
$                End of string