There might be something simpler, but this is what I came up with:
form = "C5H11BrO"
ups = c(gregexpr("[[:upper:]]", form)[[1]], nchar(form) + 1)
seperated = sapply(1:(length(ups)-1), function(x) substr(form, ups[x],
ups[x+1] - 1))
elements = gsub("[[:digit:]]", "", seperated)
nums = gsub("[[:alpha:]]", "", seperated)
ans = data.frame(element = as.character(elements),
num = as.numeric(ifelse(nums == "", 1, nums)), stringsAsFactors = FALSE)
--
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