^start of line
.*?any Unicode character (except for line terminators), repeated any number of times
:the literal : (5810 / 728 / 3A16)
\s*a Unicode whitespace character, repeated any number of times
Bthe literal B (6610 / 1028 / 4216) (case sensitive)
\s*a Unicode whitespace character, 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