insert a blank line before and after a match on mac OS

Question

my file contains the Strings:

∗Cast and characters
* Bob Denver is Gilligan

I want to insert the line * ATTENTION * to any line starting with * followed by one OR more spaces. it will be preceded by a blank line and followed by a blank line inserted above it.
I have:

/^\*[ \t]/i\
\* ATTENTION \*

it gives me

∗Cast and characters
* ATTENTION *
* Bob Denver is Gilligan 

but I want :

∗Cast and characters 

∗ ATTENTION ∗

∗ Bob Denver is Gilligan

Note: I have BSD version of sed so i can't use \n

Answer 1

you were almost there. Just need to add escaped newlines in the inserted text.

/^\*[ \t]/i\
\
\* ATTENTION \*\

However, the idiomatic way of doing it, which is posixly, portable, nd avoids backslashing is:

/^[*][[:blank:]]/!b
H;s/.*//;x
s/./&** ACHTUNG **&&/