Regular Expressions 101

(?m)applies the following effective flags to the remainder of the pattern: gm

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

^start of line

Negative Lookahead(?!.*([ACD]).*\1)

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

Group 1([ACD])

Character Class[ACD]

Athe literal A (65₁₀ / 101₈ / 41₁₆) (case sensitive)

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

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

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

\1the text saved by group 1

Negative Lookahead(?!(?>.*?B){3})

Atomic Group(?>.*?B){3}

{3}repeats the group contents below exactly 3 times:

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

Bthe literal B (66₁₀ / 102₈ / 42₁₆) (case sensitive)

Atomic Group(?>[A-D] ){2}

{2}repeats the group contents below exactly 2 times:

Character Class[A-D]

A-Done character from A (65₁₀) to D (68₁₀) (case sensitive)

the literal (32₁₀ / 40₈ / 20₁₆)

Character Class[A-D]

A-Done character from A (65₁₀) to D (68₁₀) (case sensitive)

$end of line

Global pattern flags

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

Detailed match information will be displayed here automatically.

Search reference

Regular Expression

(?m)^(?!.*([ACD]).*\1)(?!(?>.*?B){3})(?>[A-D] ){2}[A-D]$

A B C
A B B
B C B
D A B
A A B
B B B
B C C