sed issue

example.md

Input (in file input.txt)

some stuff
<fmt:message key="I_am_A"/> and some time later there is also <fmt:message key='I_am_B'/>
and even more stuff

Desired Output

I_am_A
I_am_B

---

How far I got:

grep "fmt:message" input.txt | sed -E 's/.*key=['\''"]([^'\''"]*)['\''"].*/\1/'

Which will output:

I_am_B