^start of line
Non-Capturing Group(?:.(?=.*+\n(\1?+.)))*? *?repeats the group contents below any number of times:
.any character (except for line terminators)
Positive Lookahead(?=.*+\n(\1?+.)) .*+any character (except for line terminators), repeated any number of times
\na line-feed (newline) character (ASCII 10)
\1?+optionally matches the text saved by group 1
.any character (except for line terminators)
Xthe literal X (8810 / 1308 / 5816) (case sensitive)
.*any character (except for line terminators), repeated any number of times
\na line-feed (newline) character (ASCII 10)
\1?+optionally matches the text saved by group 1
.any character (except for line terminators)
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