Regular Expressions 101

^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 (97₁₀) to z (122₁₀) (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

Group 2(fr|sa|su|mo|tu|we|th)?

?repeats the group contents below at most once:

1st Alternativefr

frthe literal text fr (case sensitive)

2nd Alternativesa

sathe literal text sa (case sensitive)

3rd Alternativesu

suthe literal text su (case sensitive)

4th Alternativemo

mothe literal text mo (case sensitive)

5th Alternativetu

tuthe literal text tu (case sensitive)

6th Alternativewe

wethe literal text we (case sensitive)

7th Alternativeth

ththe literal text th (case sensitive)

Non-Capturing Group(?:,(?:fr|sa|su|mo|tu|we|th)){0,5}

{0,5}repeats the group contents below between 0 and 5 times:

,the literal , (44₁₀ / 54₈ / 2C₁₆)

Non-Capturing Group(?:fr|sa|su|mo|tu|we|th)

1st Alternativefr

frthe literal text fr (case sensitive)

2nd Alternativesa

sathe literal text sa (case sensitive)

3rd Alternativesu

suthe literal text su (case sensitive)

4th Alternativemo

mothe literal text mo (case sensitive)

5th Alternativetu

tuthe literal text tu (case sensitive)

6th Alternativewe

wethe literal text we (case sensitive)

7th Alternativeth

ththe literal text th (case sensitive)

$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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Search reference

Regular Expression

^(?!.*\b([a-z]{2})\b.*\b\1\b)(fr|sa|su|mo|tu|we|th)?(?:,(?:fr|sa|su|mo|tu|we|th)){0,5}$

Unit Tests

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