^start of line
Positive Lookahead(?=( ?\d){5} *$) {5}repeats the group contents below exactly 5 times:
?optionally matches the literal (3210 / 408 / 2016)
\da digit
*the literal (3210 / 408 / 2016) , repeated any number of times
$end of line
\d*a digit, repeated any number of times
?optionally matches the literal (3210 / 408 / 2016)
\d*a digit, repeated any number of times
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