partition

/*
 *  call-seq:
 *     str.partition(sep)              => [head, sep, tail]
 *  
 *  Searches the string for <i>sep</i> and returns the part before it,
 *  the <i>sep</i>, and the part after it.  If <i>sep</i> is not
 *  found, returns <i>str</i> and two empty strings.  If no argument
 *  is given, Enumerable#partition is called.
 *     
 *     "hello".partition("l")         #=> ["he", "l", "lo"]
 *     "hello".partition("x")         #=> ["hello", "", ""]
 */

static VALUE
rb_str_partition(argc, argv, str)
    int argc;
    VALUE *argv;
    VALUE str;
{
    VALUE sep;
    long pos;

    if (argc == 0) return rb_call_super(argc, argv);
    rb_scan_args(argc, argv, "1", &sep);
    if (TYPE(sep) != T_REGEXP) {
        VALUE tmp;

        tmp = rb_check_string_type(sep);
        if (NIL_P(tmp)) {
            rb_raise(rb_eTypeError, "type mismatch: %s given",
                     rb_obj_classname(sep));
        }
        sep = get_arg_pat(tmp);
    }
    pos = rb_reg_search(sep, str, 0, 0);
    if (pos < 0) {
      failed:
        return rb_ary_new3(3, str, rb_str_new(0,0),rb_str_new(0,0));
    }
    sep = rb_str_subpat(str, sep, 0);
    if (pos == 0 && RSTRING(sep)->len == 0) goto failed;
    return rb_ary_new3(3, rb_str_substr(str, 0, pos),
                          sep,
                          rb_str_substr(str, pos+RSTRING(sep)->len,
                                             RSTRING(str)->len-pos-RSTRING(sep)->len));
}