How to transform a dataframe into a matrix of combinations

I have a data frame of one row that contains the results of an equation of two variables (let’s call them X and Y; for simplicity, the equation is a sum). I need to transform that dataframe into a matrix where I can see the combinations of those variables. This is a reproducible example:

x1_y1 <- 2 #Here x = 1, y = 1
x1_y2 <- 3 #Here x = 1, y = 2
x2_y1 <- 3 #Here x = 2, y = 1
x2_y2 <- 4 #Here x = 2, y = 2

df <- data.frame(x1_y1, x1_y2, x2_y1, x2_y2)

I would like the output to be something like this:

Does anybody know how to achieve this? Thanks in advance!

>Solution :

A tapply attempt after extracting the name components via strcapture:

tapply(unlist(df), strcapture("(.+)_(.+)", names(df), proto=list(x="",y="")), I)
#    y
#x    y1 y2
#  x1  2  3
#  x2  3  4