How do I make a conditional search and replace that will add a line between two lines with different match criteria?

Question

I have a text file many thousands of lines long with ASCII and non-ACII characters. It is supposed to follow a pattern of

First line: only non-ASCII characters
Second line: only non-ASCII characters
Third line: only ASCII characters
Fourth line: mix of ASCII and non-ASCII characters

Unfortunately, the reality is that it looks something like the following example, where in the middle it is missing the line that mixes ASCII and non-ASCII characters:

日本語のみ
日本語のみ
English words only
English and 日本語
日本語のみ
日本語のみ
English words only
日本語のみ
日本語のみ
English words only
English and 日本語

Fortunately, as far as I can tell, it is only the line that mixes ASCII and non-ASCII characters that is sometimes absent. Meaning that what should be groups of 4 lines are sometimes groups of only 3.

To fix the file, I need to:

1. Search for every line with only ASCII characters.
2. Test the line following to see if it contains only non-ASCII.
3. If so, insert a placeholder line following the ASCII only line.

The result should be:

日本語のみ
日本語のみ
English words only
English and 日本語
日本語のみ
日本語のみ
English words only
+Aあ+
日本語のみ
日本語のみ
English words only
English and 日本語

(I chose to make the placeholder +Aあ+ so that it will conform to the mix of ASCII and non-ASCII as the lines it is standing in for.)

I've found I can use sed to insert new lines sed -e "/this is existing text/a\'this is a new line'" < file.text. And I've learned I can search for ASCII characters with sed using LC_ALL=C and [\d0-\d127].

However, I'm unclear on how to make a conditional separate from the search. I mean, I could insert a line after every instance of ASCII only characters, but how do I make a search that inserts a line when an all ASCII line is found and the next line is only non-ASCII?

Please note that I am not particular about using sed. If an answer can be provided using Gedit, LibreOffice, or any command line operation, that would be great.

Answer 1

Based on your recent questions it sounds like you have an XY problem

Here's a sed solution based on @Zanna's answer to your previous question How do I search for lines in a file that only contain ASCII characters and then act on them?

$ LC_ALL=C sed -E '/^[\d0-\d127]+$/ {$!N; s/\n[^\d0-\d127]+$/\n+Aあ+&/;}' file
日本語のみ
日本語のみ
English words only
English and 日本語
日本語のみ
日本語のみ
English words only
+Aあ+
日本語のみ
日本語のみ
English words only
English and 日本語

Answer 2

Using awk:

awk '1; ! /^[\x01-\x7F]*$/ {next} {getline} !/[\x01-\x7F]/ {print "+Aあ+"} 1'

1. Print the input line unconditionally - 1 is a true condition, and the default action in that case is to print.
2. Then, if it isn't (!) entirely ASCII (/^[\x01-\x7F]*$/), skip processing more rules (proceeding to the next line, but processing rules from 1).
3. If it is entirely ASCII, we get the next line {getline}, and if that doesn't ! have any ASCII characters /[\x01-\x7F]/ in it, print your placeholder.
4. Finally print the line we read using getline.

I'm assuming that your 日本語のみ lines don't have half-width spaces or punctuation (. ! vs 。　！).