Using grep to check if a variable stored in a file is not undefined or blank

Question

Consider a file called .env containing:

env1=foo
env2=bar

I use grep with a regular expression to confirm there's a line defining env2 with a non-blank value, expecting to get a match.

~$ grep -c -i '^env2=(?!\s*$).+' .env
0

Returns 0 matches... but why? I got a match when I tested the same thing here: https://regexr.com/6g7of

Sanity check:

~$ grep -c -i '^env2=bar' .env
1

To confirm multiline is supported in case I had a doubt.