Regular Expressions 101

^start of line

Group 1(\S+(?:.*\b(?=[XVI])X?(?:IX|IV|V?I{0,3})\b(?= +\S))?)

\S+any non-whitespace character, repeated at least once

Non-Capturing Group(?:.*\b(?=[XVI])X?(?:IX|IV|V?I{0,3})\b(?= +\S))?

?repeats the group contents below at most once:

.*any character (except for line terminators), repeated any number of times

\ban ASCII word boundary

Positive Lookahead(?=[XVI])

Character Class[XVI]

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

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

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

X?optionally matches the literal X (88₁₀ / 130₈ / 58₁₆) (case sensitive)

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

1st AlternativeIX

IXthe literal text IX (case sensitive)

2nd AlternativeIV

IVthe literal text IV (case sensitive)

3rd AlternativeV?I{0,3}

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

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

\ban ASCII word boundary

Positive Lookahead(?= +\S)

+the literal (32₁₀ / 40₈ / 20₁₆) , repeated at least once

\Sany non-whitespace character

+the literal (32₁₀ / 40₈ / 20₁₆) , repeated at least once

Group 2(.*)

.*any character (except for line terminators), repeated any number of 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

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Search reference

Regular Expression

^(\S+(?:.*\b(?=[XVI])X?(?:IX|IV|V?I{0,3})\b(?= +\S))?) +(.*)

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