I32 maxiters;

...

    strend = s + len;
    slen = RX_MATCH_UTF8(rx) ? utf8_length((U8*)s, (U8*)strend) : len;
    maxiters = 2 * slen + 10;   /* We can match twice at each
                                   position, once with zero-length,
                                   second time with non-zero. */


...

                if (iters++ > maxiters)
                    DIE(aTHX_ "Substitution loop");