Advent of Code 2016

Recently I have encounter the webpage aka #AoC. I truly love puzzels so I was astonished with this webpage which is full of coding puzzles (24×2) . One of them (#7) I found really interesting for its solution. It was really fun to solve, because I have used regular expressions, which can be brain cracking. So here is day 7 puzzle and my solution.

Advent of Code 2016


— Day 7: Internet Protocol Version 7 —

While snooping around the local network of EBHQ, you compile a list of IP addresses (they’re IPv7, of course; IPv6 is much too limited). You’d like to figure out which IPs support TLS (transport-layer snooping).

An IP supports TLS if it has an Autonomous Bridge Bypass Annotation, or ABBA. An ABBA is any four-character sequence which consists of a pair of two different characters followed by the reverse of that pair, such as xyyx or abba. However, the IP also must not have an ABBA within any hypernet sequences, which are contained by square brackets.

For example:

abba[mnop]qrst supports TLS (abba outside square brackets).
abcd[bddb]xyyx does not support TLS (bddb is within square brackets, even though xyyx is outside square brackets).
aaaa[qwer]tyui does not support TLS (aaaa is invalid; the interior characters must be different).
ioxxoj[asdfgh]zxcvbn supports TLS (oxxo is outside square brackets, even though it’s within a larger string).
How many IPs in your puzzle input support TLS?


I have used javascript, because I was working on some project in it. First I split given data by new lines \n to get array of IPs. Then I use two regular expressions. In first I match ABBA pattern in brackets and exclude them.  And in second I match just ABBA pattern without brackets.

Now let’s look to regular expressions part by part. The second one (matchRegEx) is shorter so I will start with it. I use positive lookahead (?= ) and match my pattern ABBA with ([a-z])([a-z])\2\1, but this match AAAA pattern too. So I have to use negative lookahead (?! ) to ensures that the given pattern AAAA will not match.
The first one RegEx (inBracketsRegEx) use the same core but from both sides there are RegEx to match the pattern only in brackets. On the left side there is  \[[a-z]* to match the left square bracket and zero or more letters. For the right square bracket and zero or more letters there is  [a-z]*\].

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