Add for to the interpreter:

        if (streq(word, "for")) {
            if (cur() == '(') { pos += 1; skip(); }
            const start_val: i64 = expression();
            if (cur() == '.') { pos += 1; }
            if (cur() == '.') { pos += 1; }
            skip();
            const end_val: i64 = expression();
            if (cur() == ')') { pos += 1; skip(); }
            if (cur() == '|') { pos += 1; skip(); }
            const loop_var: []const u8 = read_name();
            if (cur() == '|') { pos += 1; skip(); }

            const body_pos: usize = pos;
            var i: i64 = start_val;
            while (i < end_val) {
                pos = body_pos;
                setVar(loop_var, i);
                _ = exec_block();
                if (return_flag or break_flag) { break; }
                i += 1;
            }
            if (break_flag) { break_flag = false; }
            if (pos <= body_pos) { pos = body_pos; skip_block(); }
            return 0;
        }

The two dots are parsed as two separate . characters. The |name| capture is the pipe characters with a variable name between them. Inside the loop, name is set as a local variable for each iteration.