regex optional string in the middle followed by negative lookahead - regex

Question

I have following entries of 3 allowed ip in a config file:
logging host 10.1.1.1
logging host ipv4 10.1.1.2
logging host 10.1.1.3
ipv4 is an optional string.I need to make sure that there are no entries with unallowed ip.For eg: if there is a line:
logging host 10.1.1.4
then the file is invalid because 10.1.1.4 is not one of the three allowed ip.I have come up with a Java regex to check for existense of any unallowed ip:
^logging host (ipv4\s)?(?!10.1.1.1|10.1.1.2|10.1.1.3)
It only works when the optional string ipv4 is not present and not when the optional string is present as in the case of second entry: "logging host ipv4 10.1.1.2".The regex engine in the first attempt greedily matches upto "logging host ipv4" and the remaining string 10.1.1.2 exists as one of the options in the negative lookahead condition.Then the regex engine makes a second attempt to non greedily match only upto "logging host" as ipv4 is optional and then remaining string becomes "ipv4 10.1.1.2" which does not exist in the negative lookahead condition and so returns this whole line as unallowed ip which is not true.
What am I missing??

Answer 1

You get a partial match because you are not matching anything after the lookahead.
For example, in logging host 10.1.1.1 the lookahead sees the value that is not allowed after matching host and there are no other options to explore so the match fails.
In logging host ipv4 10.1.1.2 the ipv4 part will be matched. Then the lookahead will see the match that is not allowed. This time it can backtrack as the ipv4 part is optional. So it can get a match from the position before ipv4, and the match is logging host
You could shorten the pattern for the specific ip numbers to 10\.1\.1\.[123]
For example
^logging host (ipv4\s)?(?!10\.1\.1\.[123])\d{1,3}(?:\.\d{1,3}){3}$
Regex demo

Answer 2

Thanks very much to 'The fourth bird' for leading me to the answer with his important hints.
To summarize I need to ensure that the config file should not contain any unallowed logging host entries.The following are allowed host entries in the config file:
logging host 10.1.1.1
logging host 10.1.1.2
logging hsot ipv6 EFD7:DEA8:AEE4::11:3
The tricky bit here is using an optional for ipv6 did not solve the problem due to backtracking at the optional:
^logging host (ipv6\s)?(?!10.1.1.1|10.1.1.2|ipv6 EFD7:DEA8:AEE4::11:3)
The first solution uses Atomic Grouping to stop backtracking and the second solution is much simpler.
^logging (?>host ipv6|host)?\s(?!10.1.1.1|10.1.1.2|EFD7:DEA8:AEE4::11:3)
^logging host\s(?!10.1.1.1|10.1.1.2|ipv6 EFD7:DEA8:AEE4::11:3)