^start of line
Non-Capturing Group(?:\d+\n)? ?repeats the group contents below at most once:
\d+a digit, repeated at least once
\na line-feed (newline) character (ASCII 10)
.*any character (except for line terminators), repeated any number of times
\h+any horizontal whitespace character, repeated at least once
\h+any horizontal whitespace character, repeated at least once
{2}matches exactly 2 characters from the set below:
0-5one character from 0 (4810) to 5 (5310)
:the literal : (5810 / 728 / 3A16)
.*any character (except for line terminators), repeated any number of times
$end of line
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