Extracting original variable list from lm object

Fellow R users,

I have an lm object, from which I would like to extract the list of
original variables.
The problem I have is the formula includes functions of the covariates.

I tried using "attr", but the result stores the transformed variable
name.  For example:

my.model<-lm(y ~ a + log(b + 1), data=my.data)
as.character(attr(my.model$terms, "variables"))[-1]

[1] "y" "a" "log(b + 1)"

But I just want a character vector of the original variables ("y", "a",
"b"), not ("y", "a", "log(b + 1)").
Does someone have a solution they could kindly share?

Thanks,
Iyue
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