How different numbers are added at the beginning of each line?

I have got this file :

...
chapter 10.
1. text
2. text
text
3. text
chapter 11.
1. text
2. text
3. text
4. text
chapter 12.
1. text
text
text
2. text
...

And I need this

...
chapter 10.
10.1. text
10.2. text
text
10.3. text
chapter 11.
11.1. text
11.2. text
11.3. text
11.4. text
chapter 12.
12.1. text
text
text
12.2. text
...

I have to put the number that comes after the chapter at the beginning of all the following lines that begin with a number followed by a dot. There are lines that do not begin with a number, those would remain unchanged. Thank you very much, any help is welcome

>Solution :

Perl to the rescue!

perl -i~ -pe '$ch = $1  if /^chapter ([0-9]+\.)/;
              print $ch if /^[0-9]+\./;
     ' -- file.txt

• -p reads the input line by line, printing each line after processing;
• -i~ saves all the changes to the original file, a backup file is created with the extension ~;
• when a line starts with chapter XX., the number of the chapter including the final dot is saved in the variable $ch;
• when the line starts with a number followed by a dot, the value from $ch is prepended.