\ba Unicode word boundary
Non-Capturing Group(?:0?[1-9]|1[0-2]) 0?optionally matches the literal 0 (4810 / 608 / 3016)
1-9one character from 1 (4910) to 9 (5710)
1the literal 1 (4910 / 618 / 3116)
0-2one character from 0 (4810) to 2 (5010)
/the literal / (4710 / 578 / 2F16)
Non-Capturing Group(?:0?[1-9]|[12]\d|3[01]) 0?optionally matches the literal 0 (4810 / 608 / 3016)
1-9one character from 1 (4910) to 9 (5710)
1the literal 1 (4910 / 618 / 3116)
2the literal 2 (5010 / 628 / 3216)
\da Unicode decimal digit
3the literal 3 (5110 / 638 / 3316)
0the literal 0 (4810 / 608 / 3016)
1the literal 1 (4910 / 618 / 3116)
/the literal / (4710 / 578 / 2F16)
Non-Capturing Group(?:19|20)? ?repeats the group contents below at most once:
\d{2}a Unicode decimal digit, repeated exactly 2 times
\ba Unicode word boundary
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