R friendly greek characters

I'm not an expert by any means but let's try to analyze the problem. In the end, your R-code needs to be understood by the compiler therefore the source-code of make.names() may be helpful:

names <- as.character(names)names2 <- .Internal(make.names(names, allow_))if (unique) {  o <- order(names != names2)  names2[o] <- make.unique(names2[o])}names2

Now, .Internal() calls the R-interpreter (written in C) so we need to go a little deeper. The C-code responsible for handling the make.names() request can be found here: https://github.com/wch/r-source/blob/0dccb93e114b00b2fcbe75e8721f11a8f2ffdff4/src/main/character.c

A short snipped:

SEXP attribute_hidden do_makenames(SEXP call, SEXP op, SEXP args, SEXP env){    SEXP arg, ans;    R_xlen_t i, n;    int l, allow_;    char *p, *tmp = NULL, *cbuf;    const char *This;    Rboolean need_prefix;    const void *vmax;    checkArity(op ,args);    arg = CAR(args);    if (!isString(arg))    error(_("non-character names"));    n = XLENGTH(arg);    allow_ = asLogical(CADR(args));    if (allow_ == NA_LOGICAL)    error(_("invalid '%s' value"), "allow_");    PROTECT(ans = allocVector(STRSXP, n));    vmax = vmaxget();    for (i = 0 ; i < n ; i++) {    This = translateChar(STRING_ELT(arg, i));    l = (int) strlen(This);    /* need to prefix names not beginning with alpha or ., as       well as . followed by a number */    need_prefix = FALSE;    if (mbcslocale && This[0]) {        int nc = l, used;        wchar_t wc;        mbstate_t mb_st;        const char *pp = This;        mbs_init(&mb_st);        used = (int) Mbrtowc(&wc, pp, MB_CUR_MAX, &mb_st);        pp += used; nc -= used;        if (wc == L'.') {        if (nc > 0) {            Mbrtowc(&wc, pp, MB_CUR_MAX, &mb_st);            if (iswdigit(wc))  need_prefix = TRUE;        }        } else if (!iswalpha(wc)) need_prefix = TRUE;    } else {        if (This[0] == '.') {        if (l >= 1 && isdigit(0xff & (int) This[1])) need_prefix = TRUE;        } else if (!isalpha(0xff & (int) This[0])) need_prefix = TRUE;    }    if (need_prefix) {        tmp = Calloc(l+2, char);        strcpy(tmp, "X");        strcat(tmp, translateChar(STRING_ELT(arg, i)));    } else {        tmp = Calloc(l+1, char);        strcpy(tmp, translateChar(STRING_ELT(arg, i)));    }    if (mbcslocale) {        /* This cannot lengthen the string, so safe to overwrite it. */        int nc = (int) mbstowcs(NULL, tmp, 0);        if (nc >= 0) {        wchar_t *wstr = Calloc(nc+1, wchar_t);        mbstowcs(wstr, tmp, nc+1);        for (wchar_t * wc = wstr; *wc; wc++) {            if (*wc == L'.' || (allow_ && *wc == L'_'))            /* leave alone */;            else if (!iswalnum((int)*wc)) *wc = L'.';        }        wcstombs(tmp, wstr, strlen(tmp)+1);        Free(wstr);        } else error(_("invalid multibyte string %d"), i+1);    } else {        for (p = tmp; *p; p++) {        if (*p == '.' || (allow_ && *p == '_')) /* leave alone */;        else if (!isalnum(0xff & (int)*p)) *p = '.';        /* else leave alone */        }    }//  l = (int) strlen(tmp);        /* needed? */    SET_STRING_ELT(ans, i, mkChar(tmp));    /* do we have a reserved word?  If so the name is invalid */    if (!isValidName(tmp)) {        /* FIXME: could use R_Realloc instead */        cbuf = CallocCharBuf(strlen(tmp) + 1);        strcpy(cbuf, tmp);        strcat(cbuf, ".");        SET_STRING_ELT(ans, i, mkChar(cbuf));        Free(cbuf);    }    Free(tmp);    vmaxset(vmax);    }    UNPROTECT(1);    return ans;}

As we can see, compiler-dependent datatypes such as wchar_t (http://icu-project.org/docs/papers/unicode_wchar_t.html) are used. This means that the behavior of make.names() depends on the C-compiler used to compile the R-interpreter itself. The problem is that C-compilers aren't very standardized therefore no assumption about the behavior of characters can be made. Everything including operating system, hardware, locale etc. can change this behavior.

In conclusion, I would stick to ASCII characters if you want to be save, especially when sharing your code between different operating systems.