I'll show you the code from regcomp.c that determines what first is.

scan = r->program + 1;  /* First BRANCH. */

Usually it is OPEN1... */
 
if (OP(scan) != BRANCH) {   /* Only one top-level choice. */
    first = scan;
    /* Skip introductions and multiplicators >= 1. */
    while ((OP(first) == OPEN && (sawopen = 1)) ||
          /* An OR of *one* alternative - should not happen now. */
        (OP(first) == BRANCH && OP(regnext(first)) != BRANCH) ||
        (OP(first) == PLUS) ||
        (OP(first) == MINMOD) ||
          /* An {n,m} with n>0 */
        (PL_regkind[(U8)OP(first)] == CURLY && ARG1(first) > 0) ) {
        if (OP(first) == PLUS)
            sawplus = 1;
        else
            first += regarglen[(U8)OP(first)];
        first = NEXTOPER(first);
    }
    /* Starting-point info. */
  again:
    if (PL_regkind[(U8)OP(first)] == EXACT) {   
        if (OP(first) == EXACT)
            ;       /* Empty, get anchored substr later. */ 
        else if ((OP(first) == EXACTF || OP(first) == EXACTFL))
            r->regstclass = first;
    }
    else if (strchr((char*)PL_simple,OP(first)))
        r->regstclass = first;
    else if (PL_regkind[(U8)OP(first)] == BOUND ||
             PL_regkind[(U8)OP(first)] == NBOUND)
        r->regstclass = first;
    else if (PL_regkind[(U8)OP(first)] == BOL) {  
        first = NEXTOPER(first);
        goto again;
    }
    else if (OP(first) == GPOS) {
        first = NEXTOPER(first);
        goto again;
    }   
    else if (!sawopen && (OP(first) == STAR &&
        PL_regkind[(U8)OP(NEXTOPER(first))] == REG_ANY) &&
        !(r->reganch & ROPT_ANCH) )
    {   
        first = NEXTOPER(first);
        goto again;
    }
          
    /* Scan is after the zeroth branch, first is atomic matcher. */
    DEBUG_r(PerlIO_printf(Perl_debug_log, "first at %"IVdf"\n",
                          (IV)(first - scan + 1)));

So it looks like you skip certain nodes until you get to a matching atom. It makes sense that /a+/ says that the atom is the "a", whereas /a*/ says that the atom is the "*", then.

What's weird, though, is /.*/ makes "." the first atom, instead of "*".

_____________________________________________________
Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
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