Regular Expressions 101

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/
(^\d{4}\d$|^\d{4}\d-\d{4}$)
/
gm
1st Capturing Group
(^\d{4}\d$|^\d{4}\d-\d{4}$)
1st Alternative
^\d{4}\d$
^ asserts position at start of a line
\d
matches a digit (equivalent to [0-9])
{4} matches the previous token exactly 4 times
\d matches a digit (equivalent to [0-9])
$ asserts position at the end of a line
2nd Alternative
^\d{4}\d-\d{4}$
^ asserts position at start of a line
\d
matches a digit (equivalent to [0-9])
{4} matches the previous token exactly 4 times
\d matches a digit (equivalent to [0-9])
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{4} matches the previous token exactly 4 times
$ 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