Regular Expressions 101

Explanation
Pro

1st AlternativeL?(?:X{0,3}(?:IX|IV|V|V?I{1,3})|IX|X{1,3})

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

Non-Capturing Group(?:X{0,3}(?:IX|IV|V|V?I{1,3})|IX|X{1,3})

1st AlternativeX{0,3}(?:IX|IV|V|V?I{1,3})

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

Non-Capturing Group(?:IX|IV|V|V?I{1,3})

1st AlternativeIX

IXthe literal text IX (case sensitive)

2nd AlternativeIV

IVthe literal text IV (case sensitive)

3rd AlternativeV

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

4th AlternativeV?I{1,3}

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

I{1,3}the literal I (73₁₀ / 111₈ / 49₁₆) (case sensitive), repeated between 1 and 3 times

2nd AlternativeIX

IXthe literal text IX (case sensitive)

3rd AlternativeX{1,3}

X{1,3}the literal X (88₁₀ / 130₈ / 58₁₆) (case sensitive), repeated between 1 and 3 times

2nd AlternativeXL

XLthe literal text XL (case sensitive)

3rd AlternativeL

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

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

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25 tokens

L?(?:X{0,3}(?:IX|IV|V|V?I{1,3})|IX|X{1,3})|XL|L

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