^            # Assert position at the beginning of the string.
(?!000|666)  # Assert that neither "000" nor "666" can be matched here.
[0-8]        # Match a digit between 0 and 8.
[0-9]{2}     # Match a digit, exactly two times.
-            # Match a literal "-".
(?!00)       # Assert that "00" cannot be matched here.
[0-9]{2}     # Match a digit, exactly two times.
-            # Match a literal "-".
(?!0000)     # Assert that "0000" cannot be matched here.
[0-9]{4}     # Match a digit, exactly four times.
$            # Assert position at the end of the string.

Apart from the ‹^› and ‹$› tokens that assert position at the beginning and end of the string, this regex can be broken into three sets of digits separated by hyphens. The first set allows any number from 000 to 899, but uses the preceding negative lookahead ‹(?!000|666)› to rule out the specific values 000 and 666. This kind of restriction can be pulled off without lookahead, but having this tool in our arsenal dramatically simplifies the regex. If you wanted to remove 000 and 666 from the range of valid area numbers without using any sort of lookaround, you’d need to restructure ‹(?!000|666)[0-8][0-9]{2}› as ‹(?:00[1-9]|0[1-9][0-9]|[1-578][0-9]{2}|6[0-57-9][0-9]|66[0-57-9])›. This far less readable approach uses a series of numeric ranges, which you can read all about in Recipe 6.7.

The second and third sets of digits in this pattern simply match any two- or four-digit number, respectively, but use a preceding negative lookahead to rule out the possibility of matching all zeros.