regex101: PCRE (Legacy) Substitution Workspace

Positive Lookahead(?=\b[MCDXLVI]{1,6}\b)

\ban ASCII word boundary

Character Class[MCDXLVI]{1,6}

{1,6}matches between 1 and 6 characters from the set below:

Mthe literal M (77₁₀ / 115₈ / 4D₁₆) (case insensitive)

Cthe literal C (67₁₀ / 103₈ / 43₁₆) (case insensitive)

Dthe literal D (68₁₀ / 104₈ / 44₁₆) (case insensitive)

Xthe literal X (88₁₀ / 130₈ / 58₁₆) (case insensitive)

Lthe literal L (76₁₀ / 114₈ / 4C₁₆) (case insensitive)

Vthe literal V (86₁₀ / 126₈ / 56₁₆) (case insensitive)

Ithe literal I (73₁₀ / 111₈ / 49₁₆) (case insensitive)

\ban ASCII word boundary

M{0,4}the literal M (77₁₀ / 115₈ / 4D₁₆) (case insensitive), repeated between 0 and 4 times

Non-Capturing Group(?:CM|CD|D?C{0,3})

1st AlternativeCM

CMthe literal text CM (case insensitive)

2nd AlternativeCD

CDthe literal text CD (case insensitive)

3rd AlternativeD?C{0,3}

D?optionally matches the literal D (68₁₀ / 104₈ / 44₁₆) (case insensitive)

C{0,3}the literal C (67₁₀ / 103₈ / 43₁₆) (case insensitive), repeated between 0 and 3 times

Non-Capturing Group(?:XC|XL|L?X{0,3})

1st AlternativeXC

XCthe literal text XC (case insensitive)

2nd AlternativeXL

XLthe literal text XL (case insensitive)

3rd AlternativeL?X{0,3}

L?optionally matches the literal L (76₁₀ / 114₈ / 4C₁₆) (case insensitive)

X{0,3}the literal X (88₁₀ / 130₈ / 58₁₆) (case insensitive), repeated between 0 and 3 times

Non-Capturing Group(?:IX|IV|V?I{0,3})

1st AlternativeIX

IXthe literal text IX (case insensitive)

2nd AlternativeIV

IVthe literal text IV (case insensitive)

3rd AlternativeV?I{0,3}

V?optionally matches the literal V (86₁₀ / 126₈ / 56₁₆) (case insensitive)

I{0,3}the literal I (73₁₀ / 111₈ / 49₁₆) (case insensitive), repeated between 0 and 3 times

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string

i modifier: insensitive. Matches without distinguishing uppercase and lowercase letters

Search reference

Regular Expression

(?=\b[MCDXLVI]{1,6}\b)M{0,4}(?:CM|CD|D?C{0,3})(?:XC|XL|L?X{0,3})(?:IX|IV|V?I{0,3})

New

Act IX: The End
Act iX: The End
Act Ix: The End
Act ix: The End
Act IX : The End
Act iX : The End
Act Ix : The End
Act ix : The End
i also me
only ii two
the iii three
what about iv

I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
XV
XVI
XVII
XVIII
XIX
XX
XLIV
XLIX
LXXIX
XCIV
XCIX
C
CI
CII
CIII
CIV
CVI
CIX
CD
CM
CC
CCC
D
DC
DCC
DCCC
CX
CXX
CXIX
CMXCIX

Regular Expressions 101

Regular Expression

Extraction