str_replace_all-with-function

library(tidyverse)

recipes <- tibble::tribble(
  ~title,          ~ingredients,
  "Banana Bread",           "4 bananas",
  "Banana Bread",      "1/2 cup butter",
  "Banana Bread",       "2/3 cup sugar",
  "Banana Bread",              "2 eggs",
  "Banana Bread",        "2 cups flour",
  "Banana Bread", "1/4 tsp baking soda"
)

                    #       🚨 IMPORTANT! 🚨
recipes %>%         #              │                   ┏ Find (number)/(number)
  mutate(           #             ┏━┓                  ┃
    ingredients_new = str_replace_all(ingredients, "\\d+/\\d+", function(x) eval(parse(text = x)))
  )                 #                                           └╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍┘
                    #                                                                    │
                    #               Evaluate as code to divide and replace in the string ┘

#> # A tibble: 6 x 3
#>   title        ingredients         ingredients_new            
#>   <chr>        <chr>               <chr>                      
#> 1 Banana Bread 4 bananas           4 bananas                  
#> 2 Banana Bread 1/2 cup butter      0.5 cup butter             
#> 3 Banana Bread 2/3 cup sugar       0.666666666666667 cup sugar
#> 4 Banana Bread 2 eggs              2 eggs                     
#> 5 Banana Bread 2 cups flour        2 cups flour               
#> 6 Banana Bread 1/4 tsp baking soda 0.25 tsp baking soda