Regular Expressions 101

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/
([12]\d{3}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01]))
/
gm
1st Capturing Group
([12]\d{3}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01]))
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])
{3} matches the previous token exactly 3 times
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
2nd 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 4510 (2D16 or 558) literally (case sensitive)
3rd 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)
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