Can not extract the capture group with neither sed nor grep

Question

I want to extract the value pair from a key-value pair syntax but I can not.
Example I tried:

echo employee_id=1234 | sed \'s/employee_id=\\([0-9]+\\)/\\1/         


        

Answer 1

To expand on anubhava's answer number 2, the general pattern to have grep return only the capture group is:

$ regex="$precedes_regex\K($capture_regex)(?=$follows_regex)"
$ echo $some_string | grep -oP "$regex"

so

# matches and returns b
$ echo "abc" | grep -oP "a\K(b)(?=c)" 
b 
# no match
$ echo "abc" | grep -oP "z\K(b)(?=c)"
# no match
$ echo "abc" | grep -oP "a\K(b)(?=d)"