Best way to test for numeric digits?

Dear List members,

What is the best way to test for numeric digits?

suppressWarnings(as.double(c("Li", "Na", "K",? "2", "Rb", "Ca", "3")))
# [1] NA NA NA? 2 NA NA? 3
The above requires the use of the suppressWarnings function. Are there 
any better ways?

I was working to extract chemical elements from a formula, something 
like this:
split.symbol.character = function(x, rm.digits = TRUE) {
 ?? ?# Perl is partly broken in R 4.3, but this works:
 ?? ?regex = "(?<=[A-Z])(?![a-z]|$)|(?<=.)(?=[A-Z])|(?<=[a-z])(?=[^a-z])";
 ?? ?# stringi::stri_split(x, regex = regex);
 ?? ?s = strsplit(x, regex, perl = TRUE);
 ?? ?if(rm.digits) {
 ?? ???? s = lapply(s, function(s) {
 ?? ???? ??? isNotD = is.na(suppressWarnings(as.numeric(s)));
 ?? ???? ??? s = s[isNotD];
 ?? ???? });
 ?? ?}
 ?? ?return(s);
}

split.symbol.character(c("CCl3F", "Li4Al4H16", "CCl2CO2AlPO4SiO4Cl"))


Sincerely,


Leonard


Note:
# works:
regex = "(?<=[A-Z])(?![a-z]|$)|(?<=.)(?=[A-Z])|(?<=[a-z])(?=[^a-z])";
strsplit(c("CCl3F", "Li4Al4H16", "CCl2CO2AlPO4SiO4Cl"), regex, perl = T)


# broken in R 4.3.1
# only slightly "erroneous" with stringi::stri_split
regex = "(?<=[A-Z])(?![a-z]|$)|(?=[A-Z])|(?<=[a-z])(?=[^a-z])";
strsplit(c("CCl3F", "Li4Al4H16", "CCl2CO2AlPO4SiO4Cl"), regex, perl = T)