Regular Expressions 101

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/
^(?:(?:0[1-9]|[12]\d|3[01])([-\/])(?:0[1-9]|1[012])\1\d{2,4}|\d{2,4}([-\/])(?:0[1-9]|1[012])\2(?:0[1-9]|[12]\d|3[01]))\s(?:[01]\d|2[0-3]):[0-5]\d$
/
gm
^ asserts position at start of a line
Non-capturing group
(?:(?:0[1-9]|[12]\d|3[01])([-\/])(?:0[1-9]|1[012])\1\d{2,4}|\d{2,4}([-\/])(?:0[1-9]|1[012])\2(?:0[1-9]|[12]\d|3[01]))
1st Alternative
(?:0[1-9]|[12]\d|3[01])([-\/])(?:0[1-9]|1[012])\1\d{2,4}
Non-capturing group
(?: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)
1st Capturing Group
([-\/])
Match a single character present in the list below
[-\/]
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
This hyphen is treated literally, which might be confusing for others. Consider escaping it or placing at the start or end of the class!
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
Non-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)
\1 matches the same text as most recently matched by the 1st capturing group
\d
matches a digit (equivalent to [0-9])
{2,4} matches the previous token between 2 and 4 times, as many times as possible, giving back as needed (greedy)
2nd Alternative
\d{2,4}([-\/])(?:0[1-9]|1[012])\2(?:0[1-9]|[12]\d|3[01])
\d
matches a digit (equivalent to [0-9])
{2,4} matches the previous token between 2 and 4 times, as many times as possible, giving back as needed (greedy)
2nd Capturing Group
([-\/])
Match a single character present in the list below
[-\/]
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
This hyphen is treated literally, which might be confusing for others. Consider escaping it or placing at the start or end of the class!
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
Non-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)
\2 matches the same text as most recently matched by the 2nd capturing group
Non-capturing group
(?: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)
\s matches any whitespace character (equivalent to [\r\n\t\f\v \u00a0\u1680\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff])
Non-capturing group
(?:[01]\d|2[0-3])
1st Alternative
[01]\d
Match a single character present in the list below
[01]
01
matches a single character in the list 01 (case sensitive)
\d matches a digit (equivalent to [0-9])
2nd Alternative
2[0-3]
2 matches the character 2 with index 5010 (3216 or 628) literally (case sensitive)
Match a single character present in the list below
[0-3]
0-3 matches a single character in the range between 0 (index 48) and 3 (index 51) (case sensitive)
: matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
Match a single character present in the list below
[0-5]
0-5 matches a single character in the range between 0 (index 48) and 5 (index 53) (case sensitive)
\d matches a digit (equivalent to [0-9])
$ asserts position at the end of a line
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

/
/
gm

Test String