Communities

Writing
Writing
Codidact Meta
Codidact Meta
The Great Outdoors
The Great Outdoors
Photography & Video
Photography & Video
Scientific Speculation
Scientific Speculation
Cooking
Cooking
Electrical Engineering
Electrical Engineering
Judaism
Judaism
Languages & Linguistics
Languages & Linguistics
Software Development
Software Development
Mathematics
Mathematics
Christianity
Christianity
Code Golf
Code Golf
Music
Music
Physics
Physics
Linux Systems
Linux Systems
Power Users
Power Users
Tabletop RPGs
Tabletop RPGs
Community Proposals
Community Proposals
tag:snake search within a tag
answers:0 unanswered questions
user:xxxx search by author id
score:0.5 posts with 0.5+ score
"snake oil" exact phrase
votes:4 posts with 4+ votes
created:<1w created < 1 week ago
post_type:xxxx type of post
Search help
Notifications
Mark all as read See all your notifications »
Users Search
Help Dashboard Sign In Sign Up
Challenges Q&A Sandbox Meta
Challenges
Posts Tags Edits
Post Challenge

Post History

66%
+2 −0
Challenges The 50 substrings that validate any string of Roman numerals

Python, 142 bytes import re r=lambda c:re.search(r'([CIMX])\1{3}|C[CM][DM]|C[DM]C|I[IX][VX]|I[VX]I|VI?[VX]|XC[CLX]|XLX|[DX]C?[DM]|[ILV][CDLM]|[ILX]X[CL]|',c)[0] almost same regex as my Per...

posted 6mo ago by jhnc‭  ·  edited 6mo ago by jhnc‭

Answer
#5: Post edited by user avatar jhnc‭ · 2026-03-11T18:09:58Z (6 months ago)
thanks __blackjack__
  • # [Python], 149 bytes
  • <!-- language-all: lang-python -->
  • import re
  • r=lambda c:re.search(r'([CIMX])\1{3}|C[CM][DM]|C[DM]C|I[IX][VX]|I[VX]I|VI?[VX]|XC[CLX]|XLX|[DX]C?[DM]|[ILV][CDLM]|[ILX]X[CL]|$',c).group(0)
  • * almost same regex as my Perl answer (added `|$` to ensure a match)
  • * increased complexity of accessing result almost entirely negates regex gain from using grouping
  • [Try it online!][TIO-mmjfmnio]
  • [Python]: https://docs.python.org/3/
  • [TIO-mmjfmnio]: https://tio.run/##XVNdi9swEHz3rxChELu4oeXeAndH2X0RrF7FguuHXM7XM@QLJ/dwNP3t6aycOKKGyCN5ZjSrVQ6fp/f97uFy6beH/XByQ1cMj5vV9uV15dbLoVscu9Wwfi@HedmQD9pWv378efh7poZC23BogTDS2Tde2yZqC4TRn6N/TlMFVewtem5YW3pOssZLbBtiGbG2Clp7/jKv19Xi97D/OJTfq1uq4@exeNsPbtPvOtfvbL44nl773bJweLbu0Q2lhf14QVK3dIuv83o@r41fVYlzGPrdqbSFxXA8Df2hrOrZt6dZvXX9m9t0u3JbuW5z7Nws/hTPs@riYZRwESekE5IJ0YR4QmFCPrNJPrHQzMVsRArKTMyFYZF7ZCYZ9tEwRjDMGyOc7t@JYoY1w1n6bGsi2xsjUEgo2N5pEzxFEDYsCKdqMowFk/eaGFoEUk3V2QszPJoyGkJFojTmB8A0UFDy91xYYRAF4pjXHMxJ5V5MEsacoDwKNLEZtv/pA8N0rKl2mBWatYFEdGwEWByCERk8TWfC7MO1K7XzJscJRSH1UQNFlXSCWrsotROCLBQcxeOKo3yNQXxG8PhFmETChcD9R/tQMKcwEcs8UhRvAc26y3bcweyCyBTOWGJmNDLBwYCW4XCwvQUX/OvoGnwKB4zl2nqQSjEkBQ5QydyhDKn7KU0msr1SoqQLaTPB/cC@DJXlm07ppr1VM@UDu7arYmK76yHp0R@mdOFNzOHqj6Vcxql1PFZ5jW637h8 "Python 3 – Try It Online"
  • # [Python], 142 bytes
  • <!-- language-all: lang-python -->
  • import re
  • r=lambda c:re.search(r'([CIMX])\1{3}|C[CM][DM]|C[DM]C|I[IX][VX]|I[VX]I|VI?[VX]|XC[CLX]|XLX|[DX]C?[DM]|[ILV][CDLM]|[ILX]X[CL]|',c)[0]
  • * almost same regex as my Perl answer (trailing `|` ensures a match)
  • * thanks to @__blackjack__‭ for the 7 byte improvement
  • [Try it online!][TIO-mmmcmbgh]
  • [Python]: https://docs.python.org/3/
  • [TIO-mmmcmbgh]: https://tio.run/##XVPBitswEL37K0QuiYsbWvYW2C5l5iIYXcWA60M262UNiRPs9LA0/fb0jZw4oobIT/J7T280yunz/HHsn67X7nA6Dmc3tMXwvN8eXt@2brcZ2vXYbofdx2pYrmryQZvy1/c/T38vVFNoag4NEEa6@NprU0dtgDD6S/Qvaaqgir1FLzVrQy9JVnuJTU0sE9ZGQWsuy2pX1t@ae5zxcyzej4Pbd33rut7m6/H81vWbwuE5uGc3rCzl71dEdBu3/rKslsvK@GWZOKeh688rW1gP43noTquyWnz9sagOrnt3@7ZfHUrX7sfWLeJP8bworx5GCRdxRjojmRHNiGcUZuQzm@QTC81czEakoMzEXBgWuUdmkmEfDWMEw7wxwunxnShmWDOcpc@2JrK9MQKFhILtnTbBUwRhw4JwqibDWDB5r4mhRSDVVJ29MMOjKaMhVCRKU34ATAMFJf/IhRUGUSCOec3BnFQexSRhzAnKk0ATm2H7nz4wTKeaKodZoVkbSESnRoDFIRiRwdN0Jsw@3LpSOW9ynFAUUh81UFRJJ6iVi1I5IchCwVE87jbK1xjEZwSPX4RJJFwIXHy0DwVzChOxzBNF8RbQrLtsxx3MLojM4YwlZkYTExwMaBkOB9tbcMHfjW7B53DAWK6sB6kUQ1LgAJXMHcqQup/SZCLbKyVKupA2E9wP7MtQWb75lO7aezVzPrAruyomtrsekh79YUoX3sQcbv5YymWcWsdTlbfoduv@AQ "Python 3 – Try It Online"
#4: Post edited by user avatar jhnc‭ · 2026-03-09T18:04:13Z (6 months ago)
  • # [Python], 149 bytes
  • <!-- language-all: lang-python -->
  • import re
  • r=lambda c:re.search(r'([CIMX])\1{3}|C[CM][DM]|C[DM]C|I[IX][VX]|I[VX]I|VI?[VX]|XC[CLX]|XLX|[DX]C?[DM]|[ILV][CDLM]|[ILX]X[CL]|$',c).group(0)
  • * almost same regex as my Perl answer (added `|$` to ensure a match)
  • * increased complexity of accessing result almost entirely negates regex gain from using grouping
  • [Try it online!][TIO-mmjfmnio]
  • [Python 3]: https://docs.python.org/3/
  • [TIO-mmjfmnio]: https://tio.run/##XVNdi9swEHz3rxChELu4oeXeAndH2X0RrF7FguuHXM7XM@QLJ/dwNP3t6aycOKKGyCN5ZjSrVQ6fp/f97uFy6beH/XByQ1cMj5vV9uV15dbLoVscu9Wwfi@HedmQD9pWv378efh7poZC23BogTDS2Tde2yZqC4TRn6N/TlMFVewtem5YW3pOssZLbBtiGbG2Clp7/jKv19Xi97D/OJTfq1uq4@exeNsPbtPvOtfvbL44nl773bJweLbu0Q2lhf14QVK3dIuv83o@r41fVYlzGPrdqbSFxXA8Df2hrOrZt6dZvXX9m9t0u3JbuW5z7Nws/hTPs@riYZRwESekE5IJ0YR4QmFCPrNJPrHQzMVsRArKTMyFYZF7ZCYZ9tEwRjDMGyOc7t@JYoY1w1n6bGsi2xsjUEgo2N5pEzxFEDYsCKdqMowFk/eaGFoEUk3V2QszPJoyGkJFojTmB8A0UFDy91xYYRAF4pjXHMxJ5V5MEsacoDwKNLEZtv/pA8N0rKl2mBWatYFEdGwEWByCERk8TWfC7MO1K7XzJscJRSH1UQNFlXSCWrsotROCLBQcxeOKo3yNQXxG8PhFmETChcD9R/tQMKcwEcs8UhRvAc26y3bcweyCyBTOWGJmNDLBwYCW4XCwvQUX/OvoGnwKB4zl2nqQSjEkBQ5QydyhDKn7KU0msr1SoqQLaTPB/cC@DJXlm07ppr1VM@UDu7arYmK76yHp0R@mdOFNzOHqj6Vcxql1PFZ5jW637h8 "Python 3 – Try It Online"
  • # [Python], 149 bytes
  • <!-- language-all: lang-python -->
  • import re
  • r=lambda c:re.search(r'([CIMX])\1{3}|C[CM][DM]|C[DM]C|I[IX][VX]|I[VX]I|VI?[VX]|XC[CLX]|XLX|[DX]C?[DM]|[ILV][CDLM]|[ILX]X[CL]|$',c).group(0)
  • * almost same regex as my Perl answer (added `|$` to ensure a match)
  • * increased complexity of accessing result almost entirely negates regex gain from using grouping
  • [Try it online!][TIO-mmjfmnio]
  • [Python]: https://docs.python.org/3/
  • [TIO-mmjfmnio]: https://tio.run/##XVNdi9swEHz3rxChELu4oeXeAndH2X0RrF7FguuHXM7XM@QLJ/dwNP3t6aycOKKGyCN5ZjSrVQ6fp/f97uFy6beH/XByQ1cMj5vV9uV15dbLoVscu9Wwfi@HedmQD9pWv378efh7poZC23BogTDS2Tde2yZqC4TRn6N/TlMFVewtem5YW3pOssZLbBtiGbG2Clp7/jKv19Xi97D/OJTfq1uq4@exeNsPbtPvOtfvbL44nl773bJweLbu0Q2lhf14QVK3dIuv83o@r41fVYlzGPrdqbSFxXA8Df2hrOrZt6dZvXX9m9t0u3JbuW5z7Nws/hTPs@riYZRwESekE5IJ0YR4QmFCPrNJPrHQzMVsRArKTMyFYZF7ZCYZ9tEwRjDMGyOc7t@JYoY1w1n6bGsi2xsjUEgo2N5pEzxFEDYsCKdqMowFk/eaGFoEUk3V2QszPJoyGkJFojTmB8A0UFDy91xYYRAF4pjXHMxJ5V5MEsacoDwKNLEZtv/pA8N0rKl2mBWatYFEdGwEWByCERk8TWfC7MO1K7XzJscJRSH1UQNFlXSCWrsotROCLBQcxeOKo3yNQXxG8PhFmETChcD9R/tQMKcwEcs8UhRvAc26y3bcweyCyBTOWGJmNDLBwYCW4XCwvQUX/OvoGnwKB4zl2nqQSjEkBQ5QydyhDKn7KU0msr1SoqQLaTPB/cC@DJXlm07ppr1VM@UDu7arYmK76yHp0R@mdOFNzOHqj6Vcxql1PFZ5jW637h8 "Python 3 – Try It Online"
#3: Post edited by user avatar jhnc‭ · 2026-03-09T17:08:03Z (6 months ago)
optimise regex
  • # [Python], 150 bytes
  • <!-- language-all: lang-python -->
  • import re
  • r=lambda c:re.findall(r'CCCC|C[CM][DM]|C[DM]C|IIII|I[IX][VX]|I[VX]I|MMMM|VI?[VX]|XC[CLX]|XLX|XXXX|[DX]C?[DM]|[ILV][CDLM]|[ILX]X[CL]|$',c)[0]
  • * same regex as my Perl answer
  • [Try it online!][TIO-mmifikdo]
  • [Python]: https://docs.python.org/3/
  • [TIO-mmifikdo]: https://tio.run/##XVPBitswEL37K0QoJCluKPQW2F3KzEUwuooB40N216EGxwl2eljwv6dv5MQRFUR@lt97eqNRLl/XP@f@1@3Wni7n4eqGphheusPp/fPgPvZDszu2/eeh6zbDmjAmqijUFYcaCDNNHmPylde6iloDYfZTwJiif0trCpHYU3RSjKlirektuVReYl0Ry4y1VnDr6du6/NhWP@tHrPFrLI7nwXVt37i2t/fdeP1s@33hME7uxQ0bpB3/viOp27vd93W5XpfG324T5zK0/XVjC7thvA7tZbMtVz9eV@XJtUfXNf3mtHVNNzZuFX@L59X25mGUcBEXpAuSBdGCeEFhQT6zST6x0MzFbEQKykzMhWGRe2QmGfbRMGYwzBsznJ7fiWKGNcNZ@mxrItsbM1BIKNjeaROMIggbFoRTNRnmgsl7TQwtAlmH0we1NwxNGQ2hIlGa8wPgNVBQ8s9cWGEQBeKY1xzMSeVZTBLGnKA8CzSxGbb/6QPDdK6pdHgrNGsDiejcCLA4BCMyeJrOhNmHe1dK502OE4pC6qMGiirpBLV0UUonBFkoOIrH5Ub5GoP4jODxizCJhAuBm4/2oWBOYSKWeaYongKadZftuIPZBZElnLHEzGhmgoMJLcPhYHsLLvjT0T34Eg4Yy6X1IJViSAocoJK5QxlS91OaTGR7pURJF9JmgvuBfRkqy7ec0kP7qGbJB3ZpV8XEdtdD0qM/TOnCm5jD3R9LuYxT63iu8h7dbt0/ "Python 3 – Try It Online"
  • # [Python], 149 bytes
  • <!-- language-all: lang-python -->
  • import re
  • r=lambda c:re.search(r'([CIMX])\1{3}|C[CM][DM]|C[DM]C|I[IX][VX]|I[VX]I|VI?[VX]|XC[CLX]|XLX|[DX]C?[DM]|[ILV][CDLM]|[ILX]X[CL]|$',c).group(0)
  • * almost same regex as my Perl answer (added `|$` to ensure a match)
  • * increased complexity of accessing result almost entirely negates regex gain from using grouping
  • [Try it online!][TIO-mmjfmnio]
  • [Python 3]: https://docs.python.org/3/
  • [TIO-mmjfmnio]: https://tio.run/##XVNdi9swEHz3rxChELu4oeXeAndH2X0RrF7FguuHXM7XM@QLJ/dwNP3t6aycOKKGyCN5ZjSrVQ6fp/f97uFy6beH/XByQ1cMj5vV9uV15dbLoVscu9Wwfi@HedmQD9pWv378efh7poZC23BogTDS2Tde2yZqC4TRn6N/TlMFVewtem5YW3pOssZLbBtiGbG2Clp7/jKv19Xi97D/OJTfq1uq4@exeNsPbtPvOtfvbL44nl773bJweLbu0Q2lhf14QVK3dIuv83o@r41fVYlzGPrdqbSFxXA8Df2hrOrZt6dZvXX9m9t0u3JbuW5z7Nws/hTPs@riYZRwESekE5IJ0YR4QmFCPrNJPrHQzMVsRArKTMyFYZF7ZCYZ9tEwRjDMGyOc7t@JYoY1w1n6bGsi2xsjUEgo2N5pEzxFEDYsCKdqMowFk/eaGFoEUk3V2QszPJoyGkJFojTmB8A0UFDy91xYYRAF4pjXHMxJ5V5MEsacoDwKNLEZtv/pA8N0rKl2mBWatYFEdGwEWByCERk8TWfC7MO1K7XzJscJRSH1UQNFlXSCWrsotROCLBQcxeOKo3yNQXxG8PhFmETChcD9R/tQMKcwEcs8UhRvAc26y3bcweyCyBTOWGJmNDLBwYCW4XCwvQUX/OvoGnwKB4zl2nqQSjEkBQ5QydyhDKn7KU0msr1SoqQLaTPB/cC@DJXlm07ppr1VM@UDu7arYmK76yHp0R@mdOFNzOHqj6Vcxql1PFZ5jW637h8 "Python 3 – Try It Online"
#2: Post edited by user avatar jhnc‭ · 2026-03-09T00:17:18Z (6 months ago)
  • # [Python], 150 bytes
  • <!-- language-all: lang-python -->
  • import re
  • r=lambda c:re.findall(r'CCCC|C[CM][DM]|C[DM]C|IIII|I[IX][VX]|I[VX]I|MMMM|VI?[VX]|XC[CLX]|XLX|XXXX|[DX]C?[DM]|[ILV][CDLM]|[ILX]X[CL]|$',c)[0]
  • [Try it online!][TIO-mmifikdo]
  • [Python]: https://docs.python.org/3/
  • [TIO-mmifikdo]: https://tio.run/##XVPBitswEL37K0QoJCluKPQW2F3KzEUwuooB40N216EGxwl2eljwv6dv5MQRFUR@lt97eqNRLl/XP@f@1@3Wni7n4eqGphheusPp/fPgPvZDszu2/eeh6zbDmjAmqijUFYcaCDNNHmPylde6iloDYfZTwJiif0trCpHYU3RSjKlirektuVReYl0Ry4y1VnDr6du6/NhWP@tHrPFrLI7nwXVt37i2t/fdeP1s@33hME7uxQ0bpB3/viOp27vd93W5XpfG324T5zK0/XVjC7thvA7tZbMtVz9eV@XJtUfXNf3mtHVNNzZuFX@L59X25mGUcBEXpAuSBdGCeEFhQT6zST6x0MzFbEQKykzMhWGRe2QmGfbRMGYwzBsznJ7fiWKGNcNZ@mxrItsbM1BIKNjeaROMIggbFoRTNRnmgsl7TQwtAlmH0we1NwxNGQ2hIlGa8wPgNVBQ8s9cWGEQBeKY1xzMSeVZTBLGnKA8CzSxGbb/6QPDdK6pdHgrNGsDiejcCLA4BCMyeJrOhNmHe1dK502OE4pC6qMGiirpBLV0UUonBFkoOIrH5Ub5GoP4jODxizCJhAuBm4/2oWBOYSKWeaYongKadZftuIPZBZElnLHEzGhmgoMJLcPhYHsLLvjT0T34Eg4Yy6X1IJViSAocoJK5QxlS91OaTGR7pURJF9JmgvuBfRkqy7ec0kP7qGbJB3ZpV8XEdtdD0qM/TOnCm5jD3R9LuYxT63iu8h7dbt0/ "Python 3 – Try It Online"
  • # [Python], 150 bytes
  • <!-- language-all: lang-python -->
  • import re
  • r=lambda c:re.findall(r'CCCC|C[CM][DM]|C[DM]C|IIII|I[IX][VX]|I[VX]I|MMMM|VI?[VX]|XC[CLX]|XLX|XXXX|[DX]C?[DM]|[ILV][CDLM]|[ILX]X[CL]|$',c)[0]
  • * same regex as my Perl answer
  • [Try it online!][TIO-mmifikdo]
  • [Python]: https://docs.python.org/3/
  • [TIO-mmifikdo]: https://tio.run/##XVPBitswEL37K0QoJCluKPQW2F3KzEUwuooB40N216EGxwl2eljwv6dv5MQRFUR@lt97eqNRLl/XP@f@1@3Wni7n4eqGphheusPp/fPgPvZDszu2/eeh6zbDmjAmqijUFYcaCDNNHmPylde6iloDYfZTwJiif0trCpHYU3RSjKlirektuVReYl0Ry4y1VnDr6du6/NhWP@tHrPFrLI7nwXVt37i2t/fdeP1s@33hME7uxQ0bpB3/viOp27vd93W5XpfG324T5zK0/XVjC7thvA7tZbMtVz9eV@XJtUfXNf3mtHVNNzZuFX@L59X25mGUcBEXpAuSBdGCeEFhQT6zST6x0MzFbEQKykzMhWGRe2QmGfbRMGYwzBsznJ7fiWKGNcNZ@mxrItsbM1BIKNjeaROMIggbFoRTNRnmgsl7TQwtAlmH0we1NwxNGQ2hIlGa8wPgNVBQ8s9cWGEQBeKY1xzMSeVZTBLGnKA8CzSxGbb/6QPDdK6pdHgrNGsDiejcCLA4BCMyeJrOhNmHe1dK502OE4pC6qMGiirpBLV0UUonBFkoOIrH5Ub5GoP4jODxizCJhAuBm4/2oWBOYSKWeaYongKadZftuIPZBZElnLHEzGhmgoMJLcPhYHsLLvjT0T34Eg4Yy6X1IJViSAocoJK5QxlS91OaTGR7pURJF9JmgvuBfRkqy7ec0kP7qGbJB3ZpV8XEdtdD0qM/TOnCm5jD3R9LuYxT63iu8h7dbt0/ "Python 3 – Try It Online"
#1: Initial revision by user avatar jhnc‭ · 2026-03-09T00:16:14Z (6 months ago) 
# [Python], 150 bytes

<!-- language-all: lang-python -->

    import re
    r=lambda c:re.findall(r'CCCC|C[CM][DM]|C[DM]C|IIII|I[IX][VX]|I[VX]I|MMMM|VI?[VX]|XC[CLX]|XLX|XXXX|[DX]C?[DM]|[ILV][CDLM]|[ILX]X[CL]|$',c)[0]

[Try it online!][TIO-mmifikdo]

[Python]: https://docs.python.org/3/
[TIO-mmifikdo]: https://tio.run/##XVPBitswEL37K0QoJCluKPQW2F3KzEUwuooB40N216EGxwl2eljwv6dv5MQRFUR@lt97eqNRLl/XP@f@1@3Wni7n4eqGphheusPp/fPgPvZDszu2/eeh6zbDmjAmqijUFYcaCDNNHmPylde6iloDYfZTwJiif0trCpHYU3RSjKlirektuVReYl0Ry4y1VnDr6du6/NhWP@tHrPFrLI7nwXVt37i2t/fdeP1s@33hME7uxQ0bpB3/viOp27vd93W5XpfG324T5zK0/XVjC7thvA7tZbMtVz9eV@XJtUfXNf3mtHVNNzZuFX@L59X25mGUcBEXpAuSBdGCeEFhQT6zST6x0MzFbEQKykzMhWGRe2QmGfbRMGYwzBsznJ7fiWKGNcNZ@mxrItsbM1BIKNjeaROMIggbFoRTNRnmgsl7TQwtAlmH0we1NwxNGQ2hIlGa8wPgNVBQ8s9cWGEQBeKY1xzMSeVZTBLGnKA8CzSxGbb/6QPDdK6pdHgrNGsDiejcCLA4BCMyeJrOhNmHe1dK502OE4pC6qMGiirpBLV0UUonBFkoOIrH5Ub5GoP4jODxizCJhAuBm4/2oWBOYSKWeaYongKadZftuIPZBZElnLHEzGhmgoMJLcPhYHsLLvjT0T34Eg4Yy6X1IJViSAocoJK5QxlS91OaTGR7pURJF9JmgvuBfRkqy7ec0kP7qGbJB3ZpV8XEdtdD0qM/TOnCm5jD3R9LuYxT63iu8h7dbt0/ "Python 3 – Try It Online"