#!/bin/bash
regex='\[assembly: AssemblyVersion\("([^"]*)"\)\]'
fileContent="foo [assembly: AssemblyVersion(\"1.2.3\")] bar\")] quux"
[[ "$fileContent" =~ $regex ]]
echo "${BASH_REMATCH[1]}"
IyEvYmluL2Jhc2gKcmVnZXg9J1xbYXNzZW1ibHk6IEFzc2VtYmx5VmVyc2lvblwoIihbXiJdKikiXClcXScKZmlsZUNvbnRlbnQ9ImZvbyBbYXNzZW1ibHk6IEFzc2VtYmx5VmVyc2lvbihcIjEuMi4zXCIpXSBiYXJcIildIHF1dXgiCltbICIkZmlsZUNvbnRlbnQiID1+ICRyZWdleCBdXQplY2hvICIke0JBU0hfUkVNQVRDSFsxXX0i