I have this string
blabla [IC=\"test\"]Hello, [IC=\"testing\"] this is a nested tag [EC], cya.[EC] blabla
I\'m trying to match the outer [
[
You could use a recursive pattern (supported in PCRE):
PCRE
\[IC[^][]*\] (?:(?:[^][]*|(?R))*) \[EC\]
See a demo on regex101.com.