Regular Expressions 101

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/
(?P<day>0[1-9]|[12]\d|3[01])(?P<seperator>[\/-])(?P<month>0[1-9]|1[012])(?P=seperator)(?P<year>\d\d\d\d)
/
gm
Named Capture Group day
(?P<day>0[1-9]|[12]\d|3[01])
1st Alternative
0[1-9]
0 matches the character 0 with index 4810 (3016 or 608) literally (case sensitive)
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]\d
Match a single character present in the list below
[12]
12
matches a single character in the list 12 (case sensitive)
\d matches a digit (equivalent to [0-9])
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)
Named Capture Group seperator
(?P<seperator>[\/-])
Match a single character present in the list below
[\/-]
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
Named Capture Group month
(?P<month>0[1-9]|1[012])
1st Alternative
0[1-9]
0 matches the character 0 with index 4810 (3016 or 608) literally (case sensitive)
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)
(?P=seperator) matches the same text as most recently matched by the capturing group named seperator
Named Capture Group year
(?P<year>\d\d\d\d)
\d matches a digit (equivalent to [0-9])
\d matches a digit (equivalent to [0-9])
\d matches a digit (equivalent to [0-9])
\d matches a digit (equivalent to [0-9])
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.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String