Extract text string in brackets

grep -oP '\(\K[^\)]+' file

\K means that use look around regex advanced feature. More precisely, it's a positive look-behind assertion, you can do it like this too :

grep -oP '(?<=\()[^\)]+' file

if you lack the -P option, you can do this with perl :

perl -lne '/\(\K[^\)]+/ and print $&' file

Another simpler approach using awk

awk -F'[()]' '{print $2}' file