use of _ .*_ and &_ in sed

Question

sed -i -e 's_.*_text-to-be-insterted-on-each-line&_' '/media/root/<filename>' 

works well and is concise and clean like i prefer.....but i dont / cant find ANY information about the ninja commands used - not in manual nor google.

• sed = Stream EDitor
  
• -i = in-place (i.e. save back to the original file)
  
• The command string:
  
  • s = the substitute command

but im having trouble understanding from example how _.*_ is translated

and how &_ is being used exactly &_ is being used

if someone could explain GREAT or any resources that document these unique sed commands?

Answer 1

s_.*_foo&_

is same as:

s/.*/foo&/

You can use other separators as well, for example:

s#.*#foo&# or s@patter@replacement@

Some time it is convenient to use separator other than / in s substitution. For example, you want to do some changes on file path.

The & in replacement part, means the matched group 0 with the given pattern (regex).

An example may better explain:

kent$  sed 's/aa/*&*/'<<<'aabbcc'
*aa*bbcc

You have seen that the matched group 0 is aa in above input string.