#!/bin/bash
s="This is the content of the textfile
version=0.4
foo=true
bar=/path/to/whatever
date=20180101
number=1
imgpath=/path/to/whatever
name=foobar"
sed '/^\(version\|date\|number\)=/!d' <<< "$s"
echo "-----"
sed -E '/^(version|date|number)=/!d' <<< "$s"