Regular Expressions 101

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"
(?P<date>((19|20)\d{2})[-/.]([1-9]|1[012])[-/.]([1-9]|[12][0-9]|3[01]))
"
gm
Named Capture Group date
(?P<date>((19|20)\d{2})[-/.]([1-9]|1[012])[-/.]([1-9]|[12][0-9]|3[01]))
2nd Capturing Group
((19|20)\d{2})
3rd Capturing Group
(19|20)
1st Alternative
19
19
matches the characters 19 literally (case sensitive)
2nd Alternative
20
20
matches the characters 20 literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{2} matches the previous token exactly 2 times
Match a single character present in the list below
[-/.]
-/.
matches a single character in the list -/. (case sensitive)
4th Capturing Group
([1-9]|1[012])
1st Alternative
[1-9]
Match a single character present in the list below
[1-9]
1-9 matches a single character in the range between 1 (index 49) and 9 (index 57) (case sensitive)
2nd Alternative
1[012]
1 matches the character 1 with index 4910 (3116 or 618) literally (case sensitive)
Match a single character present in the list below
[012]
012
matches a single character in the list 012 (case sensitive)
Match a single character present in the list below
[-/.]
-/.
matches a single character in the list -/. (case sensitive)
5th Capturing Group
([1-9]|[12][0-9]|3[01])
1st Alternative
[1-9]
Match a single character present in the list below
[1-9]
1-9 matches a single character in the range between 1 (index 49) and 9 (index 57) (case sensitive)
2nd Alternative
[12][0-9]
Match a single character present in the list below
[12]
12
matches a single character in the list 12 (case sensitive)
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
3rd Alternative
3[01]
3 matches the character 3 with index 5110 (3316 or 638) literally (case sensitive)
Match a single character present in the list below
[01]
01
matches a single character in the list 01 (case sensitive)
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