Regular Expressions 101

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"
(?ms)^\d+\.\s*(.*?)(?=^\d+\.|\Z)
"
gm
(?ms)
match the remainder of the pattern with the following effective flags: gms
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
s modifier: single line. Dot matches newline characters
^ asserts position at start of a line
\d
matches a digit (equivalent to [0-9])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v  ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
1st Capturing Group
(.*?)
.
matches any character
*? matches the previous token between zero and unlimited times, as few times as possible, expanding as needed (lazy)
Positive Lookahead
(?=^\d+\.|\Z)
Assert that the Regex below matches
1st Alternative
^\d+\.
^ asserts position at start of a line
\d
matches a digit (equivalent to [0-9])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
2nd Alternative
\Z
\Z asserts position at the end of the string, or before the line terminator right at the end of the string (if any)
Global pattern flags
g modifier: global. All matches (don't return after first match)
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
Your regular expression does not match the subject string.

Regular Expression
No Match

r"
"
gm

Test String