$ grep -Pio '(?<=heads\/)(.*?)(?=\n)' text.txt # P option instead of E
If you use GNU grep, you can use -P or --perl-regexp options.
In case you are using OS X, you need to install GNU grep.
$ brew install grep
Due to recent changes, to use GNU grep on macOS you either have to prepend the command with a ‘g’
$ ggrep -Pio '(?<=heads\/)(.*?)(?=\n)' text.txt # P option instead of E
Or change the path name