Extract value from a list of key-value pairs using grep

问题

I have a string containing a list of key-value pairs like this: "a:1,b:2,c:3". I would like to extract a value for a specified key so that e.g. I would get "1" for "a". I was planning to do it with a regex like this:

'(?<=(^|,)$KEY:)^,*'

but it seems grep doesn't support lookarounds. (I'm not even sure this regex works correctly.) Is there another way?

回答1:

You may use

grep -oP "(?:^|,)$KEY:\K[^,]+"

The -o option outputs matches. -P enables PCRE engine. The double quotes are necessary for string interpolation so that $KEY could be interpolated.

The pattern matches:

• (?:^|,) - start of string or comma
• $KEY - the KEY variable
• : - colon
• \K - match reset operator that discards the whole text matched so far
• [^,]+ - 1+ chars other than ,

回答2:

Keep it simple. You don't need all this look-a-whatever stuff, just do a simple string comparison of the field you want and then print the other field you want with awk:

$ awk -v key="a" -v RS=',' -F':' '$1==key{print $2}' <<< "a:1,b:2,c:3,"
1

That awk script will work with any awk in any shell on any UNIX box. How you pass the string to awk will be shell-dependent the <<< is a bash-ism but you can use this instead:

$ echo "a:1,b:2,c:3," | awk -v key="a" -v RS=',' -F':' '$1==key{print $2}'
1

or do other things depending if the string you want parsed is stored in a variable or a file or....

回答3:

You can use read with an IFS with colon and comma as field separators like this:

IFS=':,' read -ra arr <<< "a:1,b:2,c:3"

This will give you this array:

declare -p arr
declare -a arr=([0]="a" [1]="1" [2]="b" [3]="2" [4]="c" [5]="3")

if you want to list key-value pairs then use:

for ((i=0; i<${#arr[@]}; i+=2)); do echo "${arr[i]} => ${arr[i+1]}"; done

a => 1
b => 2
c => 3

---

To be able to fetch a single value for a given key, you may use this sed:

k=a; sed -E "s/(^|.*,)$k:([^,]*).*/\2/" <<< "a:1,b:2,c:3"
1

k=b; sed -E "s/(^|.*,)$k:([^,]*).*/\2/" <<< "a:1,b:2,c:3"
2

k=c; sed -E "s/(^|.*,)$k:([^,]*).*/\2/" <<< "a:1,b:2,c:3"
3

来源：https://stackoverflow.com/questions/51087059/extract-value-from-a-list-of-key-value-pairs-using-grep