Regular Expressions 101

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/
\d{4}\-(0?[1-9]|1[012])\-(0?[1-9]|[12][0-9]|3[01])*
/
g
\d
matches a digit (equivalent to [0-9])
{4} matches the previous token exactly 4 times
\- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
1st 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)
\- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
2nd Capturing Group
(0?[1-9]|[12][0-9]|3[01])*
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data
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)
Global pattern flags
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
g

Test String