Sed - inserting spaces

Insert a blank line after each sentence (punctuated by a carriage return)
sed G file.txt

Insert a blank line after each sentence (punctuated by a carriage return), without taking into account the existing white lines:

sed '/^$/d;G' file.txt
Add two blank lines after each sentence (punctuated by a carriage return)

sed 'G;G' fichier.txt

Insert a blank line before each line “matched” by / pattern /"(" pattern "can be a regular expression)

sed '/Fin/{x;p;x}' file.txt

Insert a blank line after each line “matched” by / pattern /"(" pattern "can be a regular expression)

sed '/Début/G' file.txt

Insert a blank line before and after each line “matched” by / pattern /"(" pattern "can be a regular expression)

sed '/Stream EDitor/{x;p;x;G}' file.txt