^start of line
Negative Lookahead(?!.{51,}) .{51,}any character (except for line terminators), repeated at least 51 times
.{1,}?any character (except for line terminators), repeated at least once
@the literal @ (6410 / 1008 / 4016)
.{4,}?any character (except for line terminators), repeated at least 4 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