Negative Lookbehind(?<!\S) \Sany character that is not Unicode whitespace
Positive Lookahead(?=\S{19,21}(?!\S)) \S{19,21}any character that is not Unicode whitespace, repeated between 19 and 21 times
\Sany character that is not Unicode whitespace
Uthe literal U (8510 / 1258 / 5516) (case sensitive)
\S*any character that is not Unicode whitespace, repeated any number of times
\d{5}a Unicode decimal digit, repeated exactly 5 times
\S*any character that is not Unicode whitespace, repeated any number of times
\da Unicode decimal digit
\Sany character that is not Unicode whitespace
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