Can not extract the capture group with either sed or grep

1. Use grep -Eo: (as egrep is deprecated)

echo 'employee_id=1234' | grep -Eo '[0-9]+'

1234

2. using grep -oP (PCRE):

echo 'employee_id=1234' | grep -oP 'employee_id=\K([0-9]+)'

1234

3. Using sed:

echo 'employee_id=1234' | sed 's/^.*employee_id=\([0-9][0-9]*\).*$/\1/'

1234