Regular Expressions 101

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/
^(19|20)\d\d[\-\/.](0[1-9]|1[012])[\-\/.](0[1-9]|[12][0-9]|3[01])$
/
^ asserts position at start of the string
1st 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])
\d matches a digit (equivalent to [0-9])
Match a single character present in the list below
[\-\/.]
\- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
2nd Capturing Group
(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)
Match a single character present in the list below
[\-\/.]
\- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
3rd Capturing Group
(0[1-9]|[12][0-9]|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][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)
$ asserts position at the end of the string, or before the line terminator right at the end of the string (if any)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/

Test String