The ASCII character classes are handled with bitmaps.

From the 5.6.1 regcomp.c, the UTF8 classes are handled with RVs. Check out S_regclassutf8(pTHX) with the code

    if (!SIZE_ONLY) {
        SV *rv = swash_init("utf8", "", listsv, 1, 0);
#ifdef DEBUGGING
        AV *av = newAV();
        av_push(av, rv);
        av_push(av, listsv);
        rv = newRV_noinc((SV*)av);
#else
        SvREFCNT_dec(listsv);
#endif
        n = add_data(1,"s");
        PL_regcomp_rx->data->data[n] = (void*)rv;
        ARG1_SET(ret, flags);
        ARG2_SET(ret, n);
    }
[download]

-Mark