Regular Expressions 101

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/
^(0[1-9]|1[0-2])\/(0[1-9]|1\d|2\d|3[01])\/(19|20)\d{2}$
/
^ asserts position at start of the string
1st Capturing Group
(0[1-9]|1[0-2])
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[0-2]
1 matches the character 1 with index 4910 (3116 or 618) literally (case sensitive)
Match a single character present in the list below
[0-2]
0-2 matches a single character in the range between 0 (index 48) and 2 (index 50) (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
2nd Capturing Group
(0[1-9]|1\d|2\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
1\d
1 matches the character 1 with index 4910 (3116 or 618) literally (case sensitive)
\d matches a digit (equivalent to [0-9])
3rd Alternative
2\d
2 matches the character 2 with index 5010 (3216 or 628) literally (case sensitive)
\d matches a digit (equivalent to [0-9])
4th 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)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
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
$ asserts position at the end of the string
Your regular expression does not match the subject string.

Regular Expression
No Match

/
/

Test String