Positive Lookahead(?=.{19,21}\s) .{19,21}any character (except for line terminators), repeated between 19 and 21 times
\sany whitespace character
Uthe literal U (8510 / 1258 / 5516) (case sensitive)
.*any character (except for line terminators), repeated any number of times
\d{5}a digit, repeated exactly 5 times
.*any character (except for line terminators), repeated any number of times
\da digit
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