Check if character value is a valid R object name
"^[.[:alpha:]][._[:alnum:]]*$"

isValidAndUnreserved <- function(string) {
    make.names(string) == string
}

isValidAndUnreserved(".jjj")
# [1] TRUE
isValidAndUnreserved(" jjj")
# [1] FALSE

isValidName <- function(string) {
    grepl("^[.[:alpha:]][._ [:alnum:]]*$", string)
}

isValidAndUnreservedName <- function(string) {
    make.names(string) == string
}

testValidity <- function(string) {
    valid <- isValidName(string)
    unreserved <- isValidAndUnreservedName(string)
    reserved <- (valid & ! unreserved)
    list("Valid"=valid,
         "Unreserved"=unreserved,
         "Reserved"=reserved)
}

testNames <- c("mean", ".j_j", "...", "if", "while", "_jj", "  j")
t(sapply(testNames, testValidity))

      Valid Unreserved Reserved
mean  TRUE  TRUE       FALSE
.j_j  TRUE  TRUE       FALSE
...   TRUE  TRUE       FALSE
if    TRUE  FALSE      TRUE
while TRUE  FALSE      TRUE
_jj   FALSE FALSE      FALSE
  j   FALSE FALSE      FALSE  # These tests for "  j", not "j"

make.names(".x")   # ".x"
make.names("_x")   # "X_x"
make.names("if")   # " if."
make.names("function")  # "function."