class SyntaxTree::CLI::Expr

would match the first expression of the input given.
An action of the CLI that outputs a pattern-matching Ruby expression that

def run(item) 

def run(item)
  program = item.handler.parse(item.source)
  if (expressions = program.statements.body) && expressions.size == 1
    puts expressions.first.construct_keys
  else
    warn("The input to `stree expr` must be a single expression.")
    exit(1)
  end
end