(?ms)applies the following effective flags to the remainder of the pattern: gms m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string
s modifier: single line. Allows a dot to match line terminators
^start of line
\s{,20}a Unicode whitespace character, repeated between 0 and 20 times
\da Unicode decimal digit
.*?any Unicode character, repeated any number of times
Positive Lookahead(?=^\s{,20}\d|\Z) 1st Alternative^\s{,20}\d ^start of line
\s{,20}a Unicode whitespace character, repeated between 0 and 20 times
\da Unicode decimal 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