What constitutes a valid symbol (identifier) in R

Question

I can’t find a spec of the language…

Note that I want a correct answer, e.g. like this, as i could easily come up with a simple, but likely wrong approximation myself, such as [[:alpha:]._][\w._]*

Answer 1

The documentation for make.names() says

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, and neither are the reserved words.

The definition of a letter depends on the current locale, but only ASCII digits are considered to be digits.

@Roland points out this section of the R language definition:

10.3.2 Identifiers

Identifiers consist of a sequence of letters, digits, the period (‘.’) and the underscore. They must not start with a digit or an underscore, or with a period followed by a digit.

The definition of a letter depends on the current locale: the precise set of characters allowed is given by the C expression (isalnum(c) || c == ‘.’ || c == ‘_’) and will include accented letters in many Western European locales.

Notice that identifiers starting with a period are not by default listed by the ls function and that ‘...’ and ‘..1’, ‘..2’, etc. are special.

Notice also that objects can have names that are not identifiers. These are generally accessed via get and assign, although they can also be represented by text strings in some limited circumstances when there is no ambiguity (e.g. "x" <- 1). As get and assign are not restricted to names that are identifiers they do not recognise subscripting operators or replacement functions.

The rules seem to allow "Morse coding":

> .__ <- 1
> ._._. <- 2
> .__ + ._._.
[1] 3