Regular Expressions 101

Pro

^start of line

Negative Lookahead(?!.*\b([A-Z]{2})\b.*\b\1\b)

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

\ban ASCII word boundary

Group 1([A-Z]{2})

Character Class[A-Z]{2}

{2}matches exactly 2 characters from the set below:

A-Zone character from A (65₁₀) to Z (90₁₀) (case sensitive)

\ban ASCII word boundary

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

\ban ASCII word boundary

\1the text saved by group 1

\ban ASCII word boundary

Non-Capturing Group(?:[A-Z]{2}(?:,\b)?)+

+repeats the group contents below at least once:

Character Class[A-Z]{2}

{2}matches exactly 2 characters from the set below:

A-Zone character from A (65₁₀) to Z (90₁₀) (case sensitive)

Non-Capturing Group(?:,\b)?

?repeats the group contents below at most once:

,the literal , (44₁₀ / 54₈ / 2C₁₆)

\ban ASCII word boundary

$end of line

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

^(?!.*\b([A-Z]{2})\b.*\b\1\b)(?:[A-Z]{2}(?:,\b)?)+$

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