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Added in 6762: Consistent port range selection for dissectors
I think the trick might be "Converts a range string" so the range needs a " (double quotes) wrapper.
The Wiki example (Using Lua to register protocols to more ports) could be rewritten as:

local tcp_port_table = DissectorTable.get("tcp.port")
local http_dissector = tcp_port_table:get_dissector(80)
tcp_port_table:add("4888-4891",http_dissector)

Search through the Wireshark source code for dissector_add_uint_range_with_preference for other range examples.

epan/wslua/wslua_dissector.c:

            /* Not a number, try as range */
            const gchar* pattern = luaL_checkstring(L,WSLUA_ARG_DissectorTable_add_PATTERN);
            range_t *range = NULL;
            if (range_convert_str(NULL, &range, pattern, G_MAXUINT32) == CVT_NO_ERROR) {
                dissector_add_uint_range(dt->name, range, handle);

epan/range.c:

/******************** Range Entry Parser *********************************/

/* Converts a range string to a fast comparable array of ranges.
 * The parameter 'es' points to the string to be converted.
 * The parameter 'max_value' specifies the maximum value in a
 * range.
 *
 * This function allocates a range_t large enough to hold the number
 * of ranges specified, and fills the array range->ranges containing
 * low and high values with the number of ranges being range->nranges.
 * After having called this function, the function value_is_in_range()
 * determines whether a given number is within the range or not.
 *
 * In case of a single number, we make a range where low is equal to high.
 * We take care on wrongly entered ranges; opposite order will be taken
 * care of.
 *
 * The following syntax is accepted :
 *
 *   1-20,30-40     Range from 1 to 20, and packets 30 to 40
 *   -20,30         Range from 1 to 20, and packet 30
 *   20,30,40-      20, 30, and the range from 40 to the end
 *   20-10,30-25    Range from 10 to 20, and from 25 to 30
 *   -              All values
 */