Using a regular expression, I need to match only the IPv4 subnet mask from the given input string:
ip=10.0.20.100::10.0.20.1:255.255.254.0:ws01.example.com::off
For testing this input string is contained in a text file called file.txt, however the actual use case will be to parse /proc/cmdline, and I will need a solution that starts parsing, counting fields, and matching after encountering "ip=" until the next white space character.
I'm using bash 4.2.46 with GNU grep 2.20 on an EL 7.9 workstation, x86_64 to test the expression.
Based on examples I've seen looking at other questions, I've come up with the following grep command and PCRE regular expression which gives output that is very close to what I need.
[user@ws01 ~]$ grep -o -P '(?<!:)(?:\:[0-9])(.*?)(?=:)' file.txt
:255.255.254.0
My understanding of what I've done here is that, I've started with a negative lookbehind with a ":" character to try and exclude the first "::" field, followed by a non capturing group to match on an escaped ":" character, followed by a number, [0-9], then a capturing group with .*?, for the actual match of the string itself, and finally a look ahead for the next ":" character.
The problem is that this gives the desired string, but includes an extra : character at the beginning of the string.
Expected output should look like this:
255.255.254.0
What's making this tricky for me to figure out is that the delimiters are not consistent. The string includes both double colons, and single colon fields, so I haven't been able to just simply match on the string between the delimiters. The reason for this is because a field can have an empty value. For example
:<null>:ip:gw:netmask:hostname:<null>:off
Null is shown here to indicate an omitted value not passed by the user, that the user does not need to provide for the intended purpose.
I've tried a few different expressions as suggested in other answers that use negative look behinds and look aheads to not start matching at a : which is neighbored by another :
For example, see this question: Regular Expression to find a string included between two characters while EXCLUDING the delimiters
If I can start matching at the first single colon, by itself, which is not followed by or preceded by another : character, while excluding the colon character as the delimiter, and continue matching until the next single colon which is also not neighboring another : and without including the colon character, that should match the desired string.
I'm able to match the exact string by including "255" in an expression like this: (Which will work for all of our present use cases)
[user@ws01 ~]$ grep -o -P '(?:)255.*?(?=:)' file.txt
255.255.254.0
The logic problem here is that the subnet mask itself, may not always start with "255", but it should be a number, [0-9] which is why I'm attempting to use that in the expression above. For the sake of simplicity, I don't need to validate that it's not greater than 255.
CodePudding user response：
Using grep
$ grep -oP '(?<!:)?:\K([0-9.] )(?=:[[:alpha:]])' file.txt
View Demo here or
$ grep -oP '[^:]*:\K[^:[:alpha:]]*' file.txt
Output
255.255.254.0
CodePudding user response：
If these are delimiters, your value should be in a clearly predictable place.
Just treat every colon as a delimiter and select the 4th field.
$: awk -F: '{print $4}' <<< ip=10.0.20.100::10.0.20.1:255.255.254.0:ws01.example.com::off
255.255.254.0
I'm not sure what you mean by
What's making this tricky for me to figure out is that the delimiters are not consistent. The string includes both double colons, and single colon fields, so I haven't been able to just simply match on the string between the delimiters.
If your delimiters aren't predictable and parse-able, they are useless. If you mean the fields can have or not have quotes, but you need to exclude quotes, we can do that. If double colons are one delimiter and single colons are another that's horrible design, but we can probably handle that, too.
$: awk -F'::' '{ split($2,x,":"); print x[2];}' <<< ip=10.0.20.100::10.0.20.1:255.255.254.0:ws01.example.com::off
255.255.254.0
For quotes, you need to provide an example.
CodePudding user response：
Using gnu-grep you could write the pattern as:
grep -oP '(?<!:):\K\d{1,3}(?:\.\d{1,3}){3}(?=:(?!:))' file.txt
Output
255.255.254.0
Explanation
(?<!:):Negative lookahead, assert not:to the left and then match:\KForget what is matched until now\d{1,3}(?:\.\d{1,3}){3}Match 4 times 1-3 digits separated by.(?=:(?!:))Positive lookahead, assert:that is not followed by:
See a regex demo.