Problem Statement

Idiomatically determine all the characters that can be used for symbols. The word symbols is meant things like names of variables, procedures (i.e., named fragments of programs, functions, subroutines, routines), statement labels, events or conditions, and in general, anything a computer programmer can choose to name, but not being restricted to this list. Identifiers might be another name for symbols. The method should find the characters regardless of the hardware architecture that is being used (ASCII, EBCDIC, or other). Display the set of all the characters that can be used for symbols which can be used (allowed) by the computer program. You may want to mention what hardware architecture is being used, and if applicable, the operating system. Note that most languages have additional restrictions on what characters can't be used for the first character of a variable or statement label, for instance. These type of restrictions needn't be addressed here (but can be mentioned).

Let's start with the solution:

# usage: gawk -f Idiomatically_determine_all_the_characters_that_can_be_used_for_symbols.awk

function is_valid_identifier(id,  rc) {
    fn = "is_valid_identifier.awk"
    printf("function unused(%s) { arr[%s] = 1 }\n", id, id, id) >fn
	printf("BEGIN { exit(0) }\n") >>fn
    close(fn)

    rc = system("gawk -f is_valid_identifier.awk 2>errors")
    return rc == 0
}

BEGIN {
    for (i = 0; i <= 255; i++) {
        c = sprintf("%c", i)

        if (is_valid_identifier(c))
            good1 = good1 c;
        else
            bad1 = bad1 c

        if (is_valid_identifier("_" c "_"))
            good2 = good2 c;
        else
            bad2 = bad2 c;
    }

    printf("1st character: %d bad, %d ok: %s\n",
        length(bad1), length(good1), good1)
    printf("2nd..nth char: %d bad, %d ok: %s\n",
        length(bad2), length(good2), good2)
    exit(0)
}