\ba Unicode word boundary
\d{6}a Unicode decimal digit, repeated exactly 6 times
:the literal : (5810 / 728 / 3A16)
.*?any Unicode character (except for line terminators), repeated any number of times
Positive Lookahead(?=\s*(?:\d{6}:|$)) \s*a Unicode whitespace character, repeated any number of times
Non-Capturing Group(?:\d{6}:|$) \d{6}a Unicode decimal digit, repeated exactly 6 times
:the literal : (5810 / 728 / 3A16)
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