\ban ASCII word boundary
\d{1}a digit, repeated exactly 1 time
\sany whitespace character
\d{3}a digit, repeated exactly 3 times
Negative Lookahead(?!\s\d{3}) \sany whitespace character
\d{3}a digit, repeated exactly 3 times
\ban ASCII 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