xxx765xxx should match
vvv123vvv should be omittedis it possible to write proper regular expression?
Yes, it's possible, but you may find it less mind-bending just to use two
regular expressions:
puts "match" if foo =~ /\A[a-z]{3}\d{3}[a-z]{3}\z/ && foo !~ /\A...123/
If you really want a single regexp, then you can explicitly code a match for
the allowed digit ranges:
re = %r{\A [a-z]{3}
([02-9]\d\d | 1[013-9]\d | 12[0124-9])
[a-z]{3} \z}x
Or you can be clever and use a negative look-ahead assertion:
re = %r{\A [a-z]{3} (?!123) \d{3} [a-z]{3} \z}x
Note that %r{...}x is just a way to write regexps with embedded spaces and
newlines, so that they can be more easily understood.
HTH,
Brian.