R `parse()` for subscripts mixed numbers and letters

Question

I am trying to display subscripts in ggplot facet labels where there are mixed letters and numbers in the subscript using parse().

This works:

>parse(text="cI[933]")

And so does this:

>parse(text="cI[W]")

But not these:

>parse(text="cI[933W]")
Error in parse(text = "cI[933W]") : <text>:1:7: unexpected symbol
1: cI[933W
          ^

> parse(text="cI[9W33]")
Error in parse(text = "cI[9W33]") : <text>:1:5: unexpected symbol
1: cI[9W33
        ^

For an example in ggplot:

data(mtcars)
mtcars$cyl2 <- factor(mtcars$cyl, labels = c("cI[123]", "cI[ABC]", "cI[456]"))
qplot(wt, mpg, data = mtcars) + facet_grid(. ~ cyl2, labeller = label_parsed)

That works fine, but:

data(mtcars)
mtcars$cyl2 <- factor(mtcars$cyl, labels = c("cI[AB3]", "cI[2CD]", "cI[EF1]"))
qplot(wt, mpg, data = mtcars) + facet_grid(. ~ cyl2, labeller = label_parsed)

...does not.

Answer 1

This is failing because 2CD is not a legal symbol according to the R parser (symbols must begin with an alphabetic character and must contain only alphanumeric + [._] characters (I think)). As a quick workaround, you can protect the illegal symbols with single quotes:

mtcars$cyl2 <- factor(mtcars$cyl, 
    labels = c("cI[AB3]", "cI['2CD']", "cI[EF1]"))

The quotation marks don't show up in the plot labels.

From the introduction to R:

Normally all alphanumeric symbols are allowed (and in some countries this includes accented letters) plus ‘.’ and ‘_’, with the restriction that a name must start with ‘.’ or a letter, and if it starts with ‘.’ the second character must not be a digit.