What is the difference between sym() and parse_expr() in the rlang package?

Question

Using the rlang package, I wonder what is the difference between sym() and parse_expr(). Consider for example the following expressions:

Answer 1

To augment the previous answer, note that ex5 and ex6 are not in fact identical.

a <- 5
b <- 3
eval_tidy(ex6)
# [1] 2
eval_tidy(ex5)
# Error in eval_tidy(ex5) : object 'a - b' not found
`a - b` <- pi
eval_tidy(ex5)
# [1] 3.141593