Regular Expressions 101

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/
(20[0-9])[\-_]([01][0-9])[\-_]([0123][0-9])
/
gm
1st Capturing Group
(20[0-9])
20 matches the characters 20 literally (case sensitive)
Match a single character present in the list below
[0-9]
{0,2} matches the previous token between 0 and 2 times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
Match a single character present in the list below
[\-_]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\- matches the character - literally (case sensitive)
_ matches the character _ literally (case sensitive)
2nd Capturing Group
([01][0-9])
Match a single character present in the list below
[01]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
01 matches a single character in the list 01 (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)
Match a single character present in the list below
[\-_]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\- matches the character - literally (case sensitive)
_ matches the character _ literally (case sensitive)
3rd Capturing Group
([0123][0-9])
Match a single character present in the list below
[0123]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
0123 matches a single character in the list 0123 (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)
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