PHP preg_replace pattern inside curly brackets, but ignoring "flags" inside square brackets

Given the following string:

Tags are inside {{ curvy }} brackets, but may also include {{ [f] flags }} in square brackets

I need to return just the "variables" – in this case "curvy" and "flags", while ignoring the flags.

I have 2 seperate regexs to achieve this, but I need the original string to remain unchanged, so I need to find a way to merge into a single regex which finds the variables, but ignores the flags.

Here are the two regexes I have now:

Grabs variables in {{ var }} –> ~{{s+(.*?)s+}}~

Grabs flags [f] OR [fdc] –> ~/[.*?]/~

The first regex works, but returns the variables with the flag.

{{(?:s*[[^][{}]*])?s*(.*?)s*}}