Regular Expressions 101

Pro

^start of line

Non-Capturing Group(?:([A-D])(?!.*?\1),)*

*repeats the group contents below any number of times:

Group 1([A-D])

Character Class[A-D]

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

Negative Lookahead(?!.*?\1)

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

\1the text saved by group 1

,the literal , (44₁₀ / 54₈ / 2C₁₆)

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

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

Detailed match information will be displayed here automatically.

25 tokens

^(?:([A-D])(?!.*?\1),)*[A-D]$

New

Some matches would be:
A
C
C,B
B,D
D,B,A,C
Some no matches would be:
A,A
ABC
D,B,A,B