/\d{2}-\d?\d{4}/
[download]

• \d matches a digit.
• {...} is a quantifier allowing you to specify an exact number of times its atom must match, or a range of legal number of times.
• - is treated as a literal hyphen in this case.
• ? as a quantifier specifies zero or one times.

It could probably be simplified to:

m/\d\d-\d{4,5}/
[download]

...since there's no real difference in this case between four digits with an optional leading extra digit, and four digits with an optional trailing extra digit.

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