\ban ASCII word boundary
\d{3}a digit, repeated exactly 3 times
?matches at most one character from the set below:
.the literal . (4610 / 568 / 2E16)
\hany horizontal whitespace character
\d{3}a digit, repeated exactly 3 times
\1the text saved by group 1
\d{2}a digit, repeated exactly 2 times
Non-Capturing Group(?:\1\d{1,2})? ?repeats the group contents below at most once:
\1the text saved by group 1
\d{1,2}a digit, repeated between 1 and 2 times
\ban ASCII word boundary
g modifier: global. Finds all matches instead of stopping after the first