Regular Expressions 101

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"
M(r|rs|s)\.?\s[A-Z]\w*
"
gm
M matches the character M with index 7710 (4D16 or 1158) literally (case sensitive)
1st Capturing Group
(r|rs|s)
1st Alternative
r
r matches the character r with index 11410 (7216 or 1628) literally (case sensitive)
2nd Alternative
rs
rs
matches the characters rs literally (case sensitive)
3rd Alternative
s
s matches the character s with index 11510 (7316 or 1638) literally (case sensitive)
\.
matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\s matches any whitespace character (equivalent to [\r\n\t\f\v  ])
Match a single character present in the list below
[A-Z]
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
\w
matches any word character (equivalent to [a-zA-Z0-9_])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
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