# Match Roman Numerals

### Challenge

Given some input string, return a truthy value if it represents a correct roman numeral between 1 (=I) and 3999 (=MMMCMXCIX), and a falsey value otherwise.

### Details

• The input is a non-empty string that only comprises the characters IVXLCDM.
• The roman numerals (that we use here in this challenge) are defined as follows:

We use only following symbols:

Symbol  I   V   X   L   C   D    M
Value   1   5  10  50 100 500 1000


To define which strings are actually valid roman numerals, it is probably easiest to provide the rule of conversation: To write a decimal number a3 a2 a1 a0 (where each ai represents one digit. So for example to represent 792 we have a3=0, a2=7, a1=9, a0=2.) as a roman numeral, we decompose it into the power of tens. The different powers of ten can be written as follows:

      1-9: I, II, III, IV, V, VI, VII, VIII, IX
10-90: X, XX, XXX, XL, L, LX, LXX, LXXX, XC
100-900: C, CC, CCC, CD, D, DC, DCC, DCCC, CM
1000-3000: M, MM, MMM


Beginning at the left side with the most significant digit of the, we can convert the number that each digit represents separately and concatenate them. So for the example from above this would look like so:

Digit        a3    a2   a1   a0
Decimal       0     7    9    2
Roman             DCC   XC   II


Therefore the roman numeral for 792 is DCCXCII. Here is a full list of all roman numerals that are relevant for this challenge: OEIS a006968.txt

### Examples

Truthy

MCCXXXIV (1234)
CMLXXXVIII (988)
DXIV (514)
CI (101)


Falsey

MMIXVIII
IVX
IXV
MMMM
XXXVX
IVI
VIV

• Subset of this conversion challenge. Commented Apr 11, 2019 at 13:03
• I still don't think this qualifies as a "subset" as the set of invalid inputs is larger. This challenge here only refers to the "well"-defined numbers that are used in OEIS A006968 Commented Apr 11, 2019 at 13:59
• Why is MMMM invalid? Is there a letter for 5000 that should be used instead for M<letter>? Commented Apr 12, 2019 at 14:14
• Check out the specs, there is no such letter. The only symbols used are I,V,X,L,C,D,M. Commented Apr 12, 2019 at 15:20

# Verbose, 1362 bytes

GET A ROMAN NUMERAL TYPED IN BY THE CURRENT PERSON USING THIS PROGRAM AND PUT IT ONTO THE TOP OF THE PROGRAM STACK
PUT THE NUMBER MMMM ONTO THE TOP OF THE PROGRAM STACK
MOVE THE FIRST ELEMENT OF THE PROGRAM STACK TO THE SECOND ELEMENT'S PLACE AND THE SECOND ELEMENT OF THE STACK TO THE FIRST ELEMENT'S PLACE
DIVIDE THE FIRST ELEMENT OF THE PROGRAM STACK BY THE SECOND ELEMENT OF THE PROGRAM STACK AND PUT THE RESULT ONTO THE TOP OF THE PROGRAM STACK
PUT THE NUMBER V ONTO THE TOP OF THE PROGRAM STACK
GET THE FIRST ELEMENT OF THE PROGRAM STACK AND THE SECOND ELEMENT OF THE PROGRAM STACK AND IF THE SECOND ELEMENT OF THE PROGRAM STACK IS NOT ZERO JUMP TO THE INSTRUCTION THAT IS THE CURRENT INSTRUCTION NUMBER AND THE FIRST ELEMENT ADDED TOGETHER'S RESULT
PUT THE NUMBER I ONTO THE TOP OF THE PROGRAM STACK
GET THE TOP ELEMENT OF THE STACK AND OUTPUT IT FOR THE CURRENT PERSON USING THIS PROGRAM TO SEE
PUT THE NUMBER III ONTO THE TOP OF THE PROGRAM STACK
GET THE FIRST ELEMENT OF THE PROGRAM STACK AND THE SECOND ELEMENT OF THE PROGRAM STACK AND IF THE SECOND ELEMENT OF THE PROGRAM STACK IS NOT ZERO JUMP TO THE INSTRUCTION THAT IS THE CURRENT INSTRUCTION NUMBER AND THE FIRST ELEMENT ADDED TOGETHER'S RESULT
PUT THE NUMBER NULLA ONTO THE TOP OF THE PROGRAM STACK
GET THE TOP ELEMENT OF THE STACK AND OUTPUT IT FOR THE CURRENT PERSON USING THIS PROGRAM TO SEE


Outputs I for valid roman numerals in the range I-MMMCMXCIX and NULLA (0) or informs user input is not a valid roman numeral otherwise.

• I can't decide if this is the right tool for the job or not. Commented Apr 11, 2019 at 17:57
• Is this the right tool for any job? Commented Apr 11, 2019 at 19:39

## C# (Visual C# Interactive Compiler), 79 109 bytes

This seems like a Regex challenge, I'm sure a shorter solution can be found...

s=>System.Text.RegularExpressions.Regex.IsMatch(s,"^M{0,3}(C[MD]|D?C{0,3})(X[CL]|L?X{0,3})(I[XV]|V?I{0,3})$")  Try it online! • Couldn't you shorten {0,3} to {,3}? Commented Apr 11, 2019 at 13:44 • @flawr doesn't seem to capture anything then Commented Apr 11, 2019 at 13:55 • Ah sorry, only things like {5,} work, but not {,5}. Commented Apr 11, 2019 at 14:01 • You can add it as compiler flag instead, so it's 72 bytes and the language should be changed to C# (Visual C# Interactive Compiler) with flag /u:System.Text.RegularExpressions.Regex, like this answer :) Commented Apr 11, 2019 at 20:07 • Alternate regex: ^M?M?M?(C[MD]|D?C?C?C?)(X[CL]|L?X?X?X?)(I[XV]|V?I?I?I?)$. Same length, but looks weirder (which is the goal, right?) Commented Apr 11, 2019 at 20:30

# Java 8, 70 bytes

s->s.matches("M{0,3}(C[MD]|D?C{0,3})(X[CL]|L?X{0,3})(I[XV]|V?I{0,3})")


Port of @Innat3's C# answer, so make sure to upvote him!

Try it online.

Explanation:

s->                // Method with String parameter and boolean return-type
s.matches("...") //  Check if the string matches the regex fully
//  (which implicitly adds a leading "^" and trailing "$") M{0,3} // No, 1, 2, or 3 adjacent "M" ( | ) // Followed by either: C[MD] // A "C" with an "M" or "D" after it | // or: D? // An optional "D" C{0,3} // Followed by no, 1, 2, or 3 adjacent "C" ( | ) // Followed by either: X[CL] // An "X" with a "C" or "L" after it | // or: L? // An optional "L" X{0,3} // Followed by no, 1, 2, or 3 adjacent "X" ( | ) // Followed by either: I[XV] // An "I" with an "X" or "V" after it | // or: V? // An optional "V" I{0,3} // Followed by no, 1, 2, or 3 adjacent "I"  # Wolfram Language (Mathematica), 35 bytes Check[FromRomanNumeral@#<3999,1<0]&  Try it online! 5 bytes saved, thanks to @attinat the limitation [1,3999] unfortunateley costs 7 bytes... here is the code for any roman number # Wolfram Language (Mathematica), 28 bytes Check[FromRomanNumeral@#,F]&  Try it online! the above code works for any number, not just [1,3999] • @ExpiredData "The input is a non-empty string that only comprises the characters IVXLCDM." Commented Apr 11, 2019 at 17:31 • 35 bytes. Boole is also shorter (by one byte) than using If in that way. – att Commented Apr 11, 2019 at 19:28 # CP-1610 assembly (Intellivision), 52 ... 48 47 DECLEs1 = 59 bytes Let's try this on a system that predates Perl by a good 7 years. :-) Takes a pointer to a null-terminated string in R4. Sets the Zero flag if the input is a valid Roman numeral, or clears it otherwise.  ROMW 10 ; use 10-bit ROM width ORG$4800           ; map this program at $4800 ;; ------------------------------------------------------------- ;; ;; test code ;; ;; ------------------------------------------------------------- ;; 4800 EIS ; enable interrupts 4801 SDBD ; R5 = pointer into test case index 4802 MVII #ndx, R5 4805 MVII #$214,    R3    ; R3 = backtab pointer
4807            MVII    #11,      R0    ; R0 = number of test cases

4809  loop      SDBD                    ; R4 = pointer to next test case
480A            MVI@    R5,       R4
480B            PSHR    R0              ; save R0, R3, R5 onto the stack
480C            PSHR    R3
480D            PSHR    R5
480E            CALL    isRoman         ; invoke our routine
4811            PULR    R5              ; restore R5 and R3
4812            PULR    R3

4813            MVII    #$1A7, R0 ; use a white 'T' by default 4815 BEQ disp 4817 MVII #$137,    R0    ; or a white 'F' is the Z flag was cleared

4819  disp      MVO@    R0,       R3    ; draw it
481A            INCR    R3              ; increment the backtab pointer

481B            PULR    R0              ; restore R0
481C            DECR    R0              ; and advance to the next test case, if any
481D            BNEQ    loop

481F            DECR    R7              ; loop forever

;; ------------------------------------------------------------- ;;
;;  test cases                                                   ;;
;; ------------------------------------------------------------- ;;
4820  ndx       BIDECLE test0, test1, test2, test3
4828            BIDECLE test4, test5, test6, test7, test8, test9, test10

; truthy
4836  test0     STRING  "MCCXXXIV", 0
483F  test1     STRING  "CMLXXXVIII", 0
484A  test2     STRING  "DXIV", 0
484F  test3     STRING  "CI", 0

; falsy
4852  test4     STRING  "MMIXVIII", 0
485B  test5     STRING  "IVX", 0
485F  test6     STRING  "IXV", 0
4863  test7     STRING  "MMMM", 0
4868  test8     STRING  "XXXVX", 0
486E  test9     STRING  "IVI", 0
4872  test10    STRING  "VIV", 0

;; ------------------------------------------------------------- ;;
;;  routine                                                      ;;
;; ------------------------------------------------------------- ;;
isRoman   PROC

4876            PSHR    R5              ; push the return address

4877            MOVR    R7,       R2    ; R2 = dummy 1st suffix
4878            MOVR    R2,       R5    ; R5 = pointer into table
4879            ADDI    #@tbl-$+1,R5 487B @loop MVI@ R5, R1 ; R1 = main digit (M, C, X, I) 487C MVI@ R5, R3 ; R3 = prefix or 2nd suffix (-, D, L, V) 487D MVI@ R4, R0 ; R0 = next digit 487E CMPR R0, R3 ; if this is the prefix ... 487F BNEQ @main 4881 COMR R2 ; ... disable the suffixes 4882 COMR R3 ; by setting them to invalid values 4883 MVI@ R4, R0 ; and read R0 again 4884 @main CMPR R0, R1 ; if R0 is not equal to the main digit, 4885 BNEQ @back ; assume that this part is over 4887 MVI@ R4, R0 ; R0 = next digit 4888 CMPR R0, R1 ; if this is a 2nd occurrence 4889 BNEQ @suffix ; of the main digit ... 488B CMP@ R4, R1 ; ... it may be followed by a 3rd occurrence 488C BNEQ @back 488E MOVR R2, R0 ; if so, force the test below to succeed 488F @suffix CMPR R0, R2 ; otherwise, it may be either the 1st suffix 4890 BEQ @next 4892 CMPR R0, R3 ; or the 2nd suffix (these tests always fail 4893 BEQ @next ; if the suffixes were disabled above) 4895 @back DECR R4 ; the last digit either belongs to the next ; iteration or is invalid 4896 @next MOVR R1, R2 ; use the current main digit ; as the next 1st suffix 4897 SUBI #'I', R1 ; was it the last iteration? ... 4899 BNEQ @loop 489B CMP@ R4, R1 ; ... yes: make sure that we've also reached ; the end of the input 489C PULR R7 ; return 489D @tbl DECLE 'M', '-' ; table format: main digit, 2nd suffix 489F DECLE 'C', 'D' 48A1 DECLE 'X', 'L' 48A3 DECLE 'I', 'V' ENDP  ### How? The regular expression can be rewritten as 4 groups with the same structure, provided that # is any invalid character that is guaranteed not be present in the input string.  +-------+---> main digit | | (M[##]|#?M{0,3})(C[MD]|D?C{0,3})(X[CL]|L?X{0,3})(I[XV]|V?I{0,3}) || | |+--+-----> prefix or second suffix | +---------> first suffix  The first suffix of the group $$\N\$$ is the main digit of the group $$\N-1\$$. Therefore, we can store the patterns with the pair $$\(\text{main_digit}, \text{second_suffix})\$$ alone. Our routine attempts to parse the input string character by character according to these patterns and eventually checks whether the end of the string is reached. ### Output screenshot of jzIntv 1. A CP-1610 opcode is encoded with a 10-bit value, known as a 'DECLE'. This routine is 47 DECLEs long, starting at$4876 and ending at $48A4 (included). • wouldn't this be one of the few places where fractional bytes are valid Commented Apr 12, 2019 at 7:34 • @ASCII-only I used to think so, but I don't know for sure. See the comments of this answer for some insight about this. Commented Apr 12, 2019 at 7:36 • @ASCII-only Also, I've just found this post in meta that tends to confirm it's probably best to round to whole bytes. Commented Apr 12, 2019 at 7:52 • ah, so it's only 10 bits when it's in RAM? Commented Apr 12, 2019 at 7:54 • The program is never stored in RAM, only in ROM. So it depends on the memory chips used in the cartridge. The CPU is designed to access either 10-bit or 16-bit ROM. The "ROMW 10" directive forces the compiler to generate code in 10-bit format. Commented Apr 12, 2019 at 7:57 # R, 7471 56 bytes Thanks to @RobinRyder, @Giuseppe, & @MickyT for their suggestions how to use grep effectively with R's built in as.roman. sub("^M(.+)","\\1",scan(,""))%in%paste(as.roman(1:2999))  Try it online! • as.roman won't work anyway, since it only works up to 3899 for some reason. Commented Apr 11, 2019 at 15:56 • I really should read the documentation better, Probably because 4000 doesn't have a definite representation in Roman, so how'd one do 3900. This is similar to 390 and now I just found an issue with my grep where I'd have to anchor the pattern. Commented Apr 11, 2019 at 16:01 • @Giuseppe, addressed, using the same regex as the other answers. Commented Apr 11, 2019 at 16:26 • 66 bytes using as.roman: first strip the initial M if there is one, then check whether the result is in as.roman(1:2999). This requires special handling of the case where the input is M. Commented Apr 11, 2019 at 17:49 • My last question is, who the heck decided that romans would be a useful thing to put into R??? It was added in 2.5.0 (April 2007)... Commented Apr 11, 2019 at 20:11 # Wolfram Language (Mathematica), 32 bytes RomanNumeral@Range@3999~Count~#&  Try it online! # Jelly, 48 47 46 44 bytes -1 thanks to Nick Kennedy 5Żo7;“ÆæC‘ð“IVXLCDM”ṃ@3Ƥm2”MẋⱮ3¤ṭŻ€ṚŒpF€ḟ€0ċ  A monadic Link accepting a non-empty list of characters consisting only of IVXLCDM which yields either 1 (when it's a valid Roman numeral between $$\1\$$ and $$\3999\$$) or 0 (if not). Try it online! Or see the test-suite. ### How? 5Żo7;“ÆæC‘ð“IVXLCDM”ṃ@3Ƥm2”MẋⱮ3¤ṭŻ€ṚŒpF€ḟ€0ċ - Main Link: list of characters S 5Żo7;“ÆæC‘ - chain 1: f(S) -> X 5Ż - zero range of five = [0,1,2,3,4,5] o7 - OR seven [7,1,2,3,4,5] “ÆæC‘ - list of code-page indices [13,22,67] ; - concatenate [7,1,2,3,4,5,13,22,67] ð - start a new dyadic chain... “IVXLCDM”ṃ@3Ƥm2”MẋⱮ3¤ṭŻ€ṚŒpF€ḟ€0ċ - chain 2: f(X,S) -> isValid “IVXLCDM” - list of characters, IVXLCDM 3Ƥ - for infixes of length three: - (i.e. IVX VXL XLC LCD CDM) ṃ@ - base decompression with swapped arguments - (i.e. use characters as base-3 digits of X's values) - (e.g. IVX -> VI I V IX II IV III VII VIII) m2 - modulo two slice (results for IVX XLC and CDM only) ¤ - nilad followed by link(s) as a nilad: ”M - character 'M' Ɱ3 - map across [1,2,3] with: ẋ - repeat -> M MM MMM ṭ - tack Ż€ - prepend a zero to each Ṛ - reverse - -- now we have the table: - 0 M MM MMM - 0 DC C D CM CC CD CCC DCC DCCC - 0 LX X L XC XX XL XXX LXX LXXX - 0 VI I V IX II IV III VII VIII Œp - Cartesian product [[0,0,0,0],...,["M","CM",0,"IV"],...] F€ - flatten €ach [[0,0,0,0],...,['M','C','M',0,'I','V'],...] ḟ€0 - filter out the zeros from €ach ["",...,"MCMIV",...] ċ - count occurrences of S  • There seems to be a redundant space on the first line. Another byte. Another byte can be saved by using a simpler first line. Try it online! Commented Apr 12, 2019 at 6:27 • Thanks, I've saved one more from it. Commented Apr 12, 2019 at 7:38 # Perl 5 (-p), 57 bytes $_=/^M*(C[MD]|D?C*)(X[CL]|L?X*)(I[XV]|V?I*)$/&!/(.)\1{3}/  TIO • uses almost the same regular expression except {0,3} quantifier was changed by * • &!/(.)\1{3}/ to ensure the same character can't occur 4 times in a row. • can't be golfed with -/(.)\1{3}/ because would give-1 for IIIIVI for example # Python 2, 81 bytes import re re.compile('M{,3}(D?C{,3}|C[DM])(L?X{,3}|X[LC])(V?I{,3}|I[VX])$').match


Try it online!

Let's look at the last part of the regex, which matching the Roman numerals up to 9 (including the empty string)

V?I{,3}|I[VX]


This has two alternatives separated by |:

• V?I{,3}: An optional V followed by up to 3 I's. This matches the empty string I,II,III, V, VI,VII,VIII.
• I[VX]: An I followed by a V or X. This matches IV and IX.

The same things with X,L,C matching the tens, with C,D,M matches the hundreds, and finally ^M{,3} allows up to 3 M's (thousands) at the start.

I tried generating the template for each trio of characters rather than writing it 3 times, but this was a lot longer.

• No need for the ^ anchor at the beginning; match already implies it matches at the beginning of the string. Commented Apr 12, 2019 at 1:28
• @ShadowRanger Thanks, I removed the ^.
– xnor
Commented Apr 12, 2019 at 1:29
• Although I think you messed up the count in the edit; should be 83, not 81. Commented Apr 12, 2019 at 1:40
• @ShadowRanger The count is 81 because the f= isn't included in the code since anonynomous functions are allowed. It's just for TIO.
– xnor
Commented Apr 12, 2019 at 1:42
• Ah, makes sense. Annoying there's no way to organize it to hide that in the header or footer, but yeah, unassigned lambdas are legal, so unassigned bound methods of compiled regex should be good too. Commented Apr 12, 2019 at 1:44

# Retina, 56 51 bytes

(.)\1{3}
0
^M*(C[MD]|D?C*)(X[CL]|L?X*)(I[XV]|V?I*)$ Port of @NahuelFouilleul's Perl 5 answer, so make sure to upvote him! Explanation: (.)\1{3} # If four adjacent characters can be found which are the same 0 # Replace it with a 0 ^...$           # Then check if the string matches the following fully:
M*             #  No or any amount of adjacent "M"
(     |    )   #  Followed by either:
C[MD]         #   A "C" with an "M" or "D" after it
|        #  or:
D?      #   An optional "D"
C*    #   Followed by no or any amount of adjacent "C"
(     |    )   #  Followed by either:
X[CL]         #   An "X" with a "C" or "L" after it
|        #  or:
L?      #   An optional "L"
X*    #   Followed by no or any amount of adjacent "X"
(     |    )   #  Followed by either:
I[XV]         #   An "I" with an "X" or "V" after it
|        #  or:
V?      #   An optional "V"
I*    #   Followed by no or any amount of adjacent "I"


# 05AB1E, 619 8 bytes

ŽF¯L.XIå


Whopping $$\\color{green}{\textrm{-52 bytes}}\$$ thanks to @Adnan, because apparently 05AB1E's Roman Number builtin wasn't documented, haha.. xD

Explanation:

ŽF¯       # Push comressed integer 3999
L      # Create a list in the range [1,3999]
.X    # Convert each integer in this list to a roman number string
Iå  # Check if the input is in this list
# (and output the result implicitly)


See this 05AB1E tip of mine (section How to compress large integers?) to understand why ŽF¯ is 3999.

•1∞Γ'иÛnuÞ\₂…•Ž8вв€SÐ)v.•6#&‘нδ•u3ôNèyè}'M3L×)Rεõš}3Fâ}€˜JIå


Explanation:

•1∞Γ'иÛnuÞ\₂…•             '# Push compressed integer 397940501547566186191992778
Ž8в           # Push compressed integer 2112
в          # Convert the integer to Base-2112 as list:
#  [1,11,111,12,2,21,211,2111,10]
€S                          # Convert each number to a list of digits
Ð                         # Triplicate this list
)                        # And wrap it into a list of lists (of lists)
v                       # Loop y over each these three lists:
.•6#&‘нδ•              #  Push compressed string "xivcxlmcd"
u             #  Uppercased
3ô           #  And split into parts of size 3: ["XIV","CXL","MCD"]
Nè                     #  Use the loop index to get the current part
yè                   #  And index the list of lists of digits into this string
}'M                    '# After the loop: push "M"
3L                   # Push list [1,2,3]
×                  # Repeat the "M" that many times: ["M","MM","MMM"]
)                 # Wrap all lists on the stack into a list:
# [[["I"],["I","I"],["I","I","I"],["I","V"],["V"],["V","I"],["V","I","I"],["V","I","I","I"],["I","X"]],[["X"],["X","X"],["X","X","X"],["X","L"],["L"],["L","X"],["L","X","X"],["L","X","X","X"],["X","C"]],[["C"],["C","C"],["C","C","C"],["C","D"],["D"],["D","C"],["D","C","C"],["D","C","C","C"],["C","M"]],["M","MM","MMM"]]
R                # Reverse this list
εõš}            # Prepend an empty string "" before each inner list
# Push the four lists onto the stack
3F         # Loop 3 times:
â        #  Take the cartesian product of the two top lists
}€˜     # After the loop: flatten each inner list
J    # Join each inner list together to a single string
Iå  # And check if the input is in this list
# (after which the result is output implicitly)

• •1∞Γ'иÛnuÞ\₂…• is 397940501547566186191992778
• Ž8в is 2112
• •1∞Γ'иÛnuÞ\₂…•Ž8вв is [1,11,111,12,2,21,211,2111,10]
• .•6#&‘нδ• is "xivcxlmcd"
• I'm not sure why .X is not documented, but I think this should work: 3999L.XQO Commented Apr 13, 2019 at 20:56
• @Adnan Haha, -52 bytes right there. Completely forgot you indeed told us about adding a Roman Number builtin. Will ask @Mr.Xcoder in chat to add it to the docs. Are any other commands missing? ;) PS: Saved another byte by compressing 3999. :) Commented Apr 13, 2019 at 21:24

$_=/^$RE{num}{roman}$/&!/(.)\1{3}/  The &!/(.)\1{3}/ part is necessary, because Regexp::Common allows four (but not five) of the same characters in a row. That way, it matches roman numbers used on clock faces, where IIII is often used for 4. # Python 3, 116113109107105 106 bytes import re lambda n:re.match(r'(M{,3}(C(M|CC?|D)?|DC{,3}))(X(C|XX?|L)?|(LX{,3}))?(I(X|II?|V)?|VI{,3})?$',n)


Try it online!

• As I mentioned on the Py2 answer, the leading ^ is unnecessary since match only matches at the beginning of a string already. Commented Apr 12, 2019 at 1:35
• @ShadowRanger added anchors while debugging and then didn't try again without them. I'll remember that now - thanks! :) Commented Apr 12, 2019 at 12:57
• Well, just to be clear, the trailing $ is necessary (only fullmatch implies anchors on both ends, and obviously that would cost more than a $). Commented Apr 12, 2019 at 13:53
• @ShadowRanger Ah! That explains why I needed anchors! Didn't realize I only needed to anchor the end. Thanks again. Commented Apr 12, 2019 at 14:12

# Ruby, (-n) 56 bytes

p~/^M{,3}(D?C{,3}|CM|CD)(L?X{,3}|XC|XL)(V?I{,3}|IV|IX)\$/


Try it online!

Outputs 0 (truthy) or nil (falsy).