Regular Expressions 101

Explanation
Pro

(?m-s)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

s modifier: -single line. A dot does not match line terminators

^start of line

Group 1(..)

.any character (except for line terminators)

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

Atomic Group(?>\r?\n\1.*){2}

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

\r?optionally matches a carriage return (ASCII 13)

\na line-feed (newline) character (ASCII 10)

\1the text saved by group 1

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

\Kresets the starting point of the reported match. Any previously consumed characters are no longer included in the final match

Non-Capturing Group(?:\r?\n\1.*)+

+repeats the group contents below at least once:

\r?optionally matches a carriage return (ASCII 13)

\na line-feed (newline) character (ASCII 10)

\1the text saved by group 1

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

Global pattern flags

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

Detailed match information will be displayed here automatically.

25 tokens

(?m-s)^(..).*+(?>\r?\n\1.*){2}\K(?:\r?\n\1.*)+

at58
at59
at60
at61
at62
au344
au347
au348
au349
au350
ba5
ba6
ba7
be72
be73
be74
be75
be76

at58
at59
at60
at61
at62
au344
au347
au348
au349
au350
ba5
ba6
ba7
be72
be73
be74
be75
be76