Regular Expressions 101

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/
^[A-Z\d](?:(, | )(?:[A-Z\d]\1)*[A-Z\d] )?$
/
gm
^ asserts position at start of a line
Match a single character present in the list below
[A-Z\d]
{6} matches the previous token exactly 6 times
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
\d matches a digit (equivalent to [0-9])
Non-capturing group
(?:(, | )(?:[A-Z\d]\1)*[A-Z\d] )?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
1st Capturing Group
(, | )
1st Alternative
,
, matches the character , literally (case sensitive)
matches the character literally (case sensitive)
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
2nd Alternative
matches the character literally (case sensitive)
Non-capturing group
(?:[A-Z\d]\1)*
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below
[A-Z\d]
\1 matches the same text as most recently matched by the 1st capturing group
Match a single character present in the list below
[A-Z\d]
{6} matches the previous token exactly 6 times
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
\d matches a digit (equivalent to [0-9])
matches the character literally (case sensitive)
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
$ asserts position at the end of a line
Global pattern flags
g modifier: global. All matches (don't return after first match)
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String