Postgres group rows based on data in one row

I am trying to group rows on a postgres query. I have one row of data that has a field with a line number in it. The next 3 rows are null. I want to set them to the same value as the first row. Then the 5th row repeats the process. There may be 1 or more rows with null as the line number.

Here is a sample of the data with a simple query to pull the rows I want, ordered by line_num:

I want group_line_num to be 7 where it is null in the first set and 52 on the 4th null. The groups can also be created based on where the unit changes with the PST^ being the defining line of the group. I have tried using lag but since the number of null lines is variable I am not sure how to make it work.

The described results can be easily achieved using MAX as a window function.

The following code establishes a table of example values:

CREATE TABLE examples (
  item_id TEXT NOT NULL,
  ordered_qty INTEGER NOT NULL,
  unit TEXT NOT NULL,
  line_num INTEGER NOT NULL,
  group_line_num INTEGER,
  code TEXT
);

INSERT INTO
  examples (
    item_id,
    ordered_qty,
    unit,
    line_num,
    group_line_num,
    code
  )
VALUES
  ('NP4484140T', 11, 'PST^', 7, 7, ''),
  ('COL48LPT', 6, 'PST', 8, NULL, 'NP4484140T'),
  ('COL48EPT', 4, 'PST', 9, NULL, 'NP4484140T'),
  ('COL48BPT', 1, 'PST', 10, NULL, 'NP4484140T'),
  ('VP5578135T', 1, 'PST^', 52, 52, ''),
  ('ONTP48CPT', 1, 'PST', 53, NULL, 'VP5578135T');

To determine the group_line_num using it's most recent non-NULL value, run the following:

SELECT
  item_id,
  ordered_qty,
  unit,
  line_num,
  MAX(group_line_num) OVER (ORDER BY line_num) AS group_line_num,
  code
FROM
  examples
ORDER BY
  line_num;

The following query derives group_line_num from the most recent line_num associated with unit = 'PST^':

SELECT
  item_id,
  ordered_qty,
  unit,
  line_num,
  MAX(line_num) FILTER (WHERE unit = 'PST^') OVER (ORDER BY line_num) AS group_line_num,
  code
FROM
  examples
ORDER BY
  line_num;

The two queries return identical results: