Solving Problem: Validating Roman Numbers

Using Regex Pattern to validate if a Roman number is valid

Problem Statement: Validate if a roman number is between 1 and 3999

Solution:

regex_pattern = r'^M{0,3}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$'	# Do not delete 'r'.
import re
print(str(bool(re.match(regex_pattern, input()))))

This is the classic Roman numeral validation regex (for 1–3999)

Roman Numerals

Roman numerals originated in ancient Rome and remained widely used through the Middle Ages. They rely on fixed-value symbols rather than positional notation.

Core Symbols

• I = 1
• V = 5
• X = 10
• L = 50
• C = 100
• D = 500
• M = 1000

Additive Rule

Place symbols from largest to smallest left to right:

• VIII = 5 + 1 + 1 + 1 = 8
• LXX = 50 + 10 + 10 = 70

Subtractive Rule

Place a smaller symbol before a larger one to subtract:

• IV = 4
• IX = 9
• XL = 40
• CM = 900

Common Conversions

Regex for validating Roman Numerals from 1-3999:

regex = r’^M{0,3}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$’

Breakdown:

^

Start of string.

M{0,3}

""     = 0
M      = 1000
MM     = 2000
MMM    = 3000

(CM|CD|D?C{0,3})

Hundreds place:

CM     = 900
CD     = 400
D?C{0,3}

D?C{0,3} covers:

""     = 0
C      = 100
CC     = 200
CCC    = 300
D      = 500
DC     = 600
DCC    = 700
DCCC   = 800

(XC|XL|L?X{0,3})

XC     = 90
XL     = 40
L?X{0,3}

L?X{0,3} covers:

""     = 0
X      = 10
XX     = 20
XXX    = 30
L      = 50
LX     = 60
LXX    = 70
LXXX   = 80

(IX|IV|V?I{0,3})

Ones place:

IX     = 9
IV     = 4
V?I{0,3}

covers:

""     = 0
I      = 1
II     = 2
III    = 3
V      = 5
VI     = 6
VII    = 7
VIII   = 8

$

End of string.

Valid Matches

I
IV
IX
X
XL
XC
CD
CM
MCMXC
MCMXCIV
MMMCMXCIX   (3999)

Invalid Matches:

IIII
VV
IC
XM
MMMM
VX
IL
MCMC

References

Validating Roman Numerals | HackerRank

Python Regular Expressions  |  Python Education  |  Google for Developers