Question

Several months ago I asked something similar, but I was using JavaScript to check if provided string is a “valid” R object name. Now I’d like to achieve the same by using nothing but R. I suppose that there’s a very nice way to do this, with some neat (not so) esoteric R function, so regular expressions seem to me as the last line of defence. Any ideas?

Oh, yeah, using back-ticks and stuff is considered cheating. =)

Answer 1

Edited 2013-1-9 to fix regular expression. Previous regular expression, lifted from page 456 of John Chambers’ "Software for Data Analysis", was (subtly) incomplete. (h.t. Hadley Wickham)

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There are a couple of issues here. A simple regular expression can be used to identify all syntactically valid names — but some of those names (like if and while) are ‘reserved’, and cannot be assigned to.

• Identifying syntactically valid names:

  ?make.names explains that a syntactically valid name:

  […] consists of letters, numbers and the
  dot or underline characters and starts with a letter or the dot
  not followed by a number. Names such as ‘".2way"’ are not valid […]

  Here is the corresponding regular expression:

   "^((([[:alpha:]]|[.][._[:alpha:]])[._[:alnum:]]*)|[.])$"
  

• Identifying unreserved syntactically valid names

  To identify unreserved names, you can take advantage of the base function make.names(), which constructs syntactically valid names from arbitrary character strings.

  isValidAndUnreserved <- function(string) {
      make.names(string) == string
  }
  
  isValidAndUnreserved(".jjj")
  # [1] TRUE
  isValidAndUnreserved(" jjj")
  # [1] FALSE
  

• Putting it all together

  isValidName <- function(string) {
      grepl("^((([[:alpha:]]|[.][._[: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", "TRUE", "NULL",
                 "_jj", "  j", ".2way") 
  t(sapply(testNames, testValidity))
  

        Valid Unreserved Reserved
  mean  TRUE  TRUE       FALSE   
  .j_j  TRUE  TRUE       FALSE
  .     TRUE  TRUE       FALSE     
  ...   TRUE  TRUE       FALSE   
  if    TRUE  FALSE      TRUE    
  while TRUE  FALSE      TRUE    
  TRUE  TRUE  FALSE      TRUE    
  NULL  TRUE  FALSE      TRUE    
  _jj   FALSE FALSE      FALSE   
    j   FALSE FALSE      FALSE   # Note: these tests are for "  j", not "j"
  .2way FALSE FALSE      FALSE
  

For more discussion of these issues, see the r-devel thread linked to by @Hadley in the comments below.