^start of line
\d{4}a digit, repeated exactly 4 times
-the literal - (4510 / 558 / 2D16)
\d{3,4}a digit, repeated between 3 and 4 times
-the literal - (4510 / 558 / 2D16)
2the literal 2 (5010 / 628 / 3216)
3the literal 3 (5110 / 638 / 3316)
$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