Non-Capturing Group(?:^.*?)? ?repeats the group contents below at most once:
^start of line
.*?any character (except for line terminators), repeated any number of times
\[the literal [ (9110 / 1338 / 5B16)
@the literal @ (6410 / 1008 / 4016)
.+?any character (except for line terminators), repeated at least once
\]the literal ] (9310 / 1358 / 5D16)
.*?any character (except for line terminators), repeated any number of times
Positive Lookahead(?=\[|$) \[the literal [ (9110 / 1338 / 5B16)
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