/\d{2}-\d?\d{4}/
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• \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}/
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...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.

perlrequick, perlintro, Re^3: My Favourite Regex Tools (Was: Parsing a Variable Format String)

#~ two digits,
#~ followed by a dash,
#~ followed by an optional digit,
#~ followed by four digits 
use Regexp::English;
my $re = Regexp::English
                -> new
                -> digit
                -> digit
                -> literal('-')
                -> optional
                    -> digit
                -> end
                -> digit
                -> digit
                -> digit
                -> digit;
print $re;
__END__
(?^:\d\d\-(?:\d?)\d\d\d\d)
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rxrx describes the regex

\d             Match a digit

\d             Match a digit

\-             Match a literal '-' character

(?:            The start of a non-capturing group

  \d?            Match a digit, one-or-zero times (as many as possible
+)

)              The end of non-capturing group

\d             Match a digit

\d             Match a digit

\d             Match a digit

\d             Match a digit
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wxPPIxregexplain.pl / ppixregexplain.pl says