^start of string
.*?any Unicode character (except for line terminators), repeated any number of times
Non-Capturing Group(?:\s(?<trans>\[.*?\]))? ?repeats the group contents below at most once:
\sa Unicode whitespace character
Group trans(?<trans>\[.*?\]) \[the literal [ (9110 / 1338 / 5B16)
.*?any Unicode character (except for line terminators), repeated any number of times
\]the literal ] (9310 / 1358 / 5D16)
\sa Unicode whitespace character
—the literal — (821210 / 200248 / 201416)
\sa Unicode whitespace character
Group translate(?<translate>.*?) .*?any Unicode character (except for line terminators), repeated any number of times
$end of string, or before its final line terminator
u modifier: unicode. Treats the pattern and subject as UTF-16