sed introduce newline before a matched line

i want to transform this:

# blah
# gen = 0
1 1 1
1 1 1
# gen = 1
2 2 2
2 2 2
# gen = 2
3 3 3 3
3 3
.
.
.
# gen = 10
10 10 10

in to this :

# blah

# gen = 0
1 1 1
1 1 1

# gen = 1
2 2 2
2 2 2

# gen = 2
3 3 3 3
3 3
.
.
.

# gen = 10
10 10 10

that is i want to introduce one or more new line before all lines starting with “# gen = “.  this is how one can do it using sed.

sed 's/^# gen.*/\n\n&/' <a.out  >b.out

• ‘s/ bit  :: is for the substitute command
• ^# gen :: is the regular expression which matches all lines starting with a # followed by gen
• .* ::  matches everything else on the lines starting with a # followed by gen
• \n\n :: will introduce 2 blank lines
• & :: simply stands for the matched part that is, it stands for everything matched by ^# gen.*
• a.out :: input file the file i want to transform
• b.out :: is the required file..the file i want to have

i wish i could do the same thing sitting in emacs.

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