input: Write a function to evaluate a given numerical expression in string format and return the result. Ensure that the function handles basic arithmetic operations like addition, subtraction, multiplication, and division. If the input string is empty, return 0.
  
  def eval_expr(expr):
     if not expr:
         return 0
     return eval(expr)
  
  assert eval_expr("3+5*2-4/2")==11
  assert eval_expr("10/5+3*2")==8
  assert eval_expr("2*6+3-9/3")==12
  assert eval_expr("15+5/2-3*4")==3
  assert eval_expr("")==0
output: def eval_expr(expr):
     if not expr:
         return 0
     return eval(expr)