AWK – Add and Number Blank Line Above Each Line

If the pattern cannot occur on consecutive lines you can simply run

sed '$!N;/.*\n.*PATTERN.*/P;D' infile

I've explained here how the N;P;D cycle works. The difference is that here the first line in the pattern space is printed only if the second one matches, otherwise it's deleted.

If the pattern can occur on consecutive lines the above solution will print a line that matches if it's followed by another line that matches.
To ignore consecutive matches add a second condition to print the first line in the pattern space only if it doesn't match:

sed '$!N;/.*\n.*PATTERN.*/{/.*PATTERN.*\n.*/!P;};D' infile

Another way, using the hold buffer.
If you want to ignore consecutive matches:

sed '/PATTERN/!{              # if line doesn't match PATTERN
h                             # copy pattern space content over the hold buffer
d                             # delete pattern space
}
//{                           # if line matches PATTERN
x                             # exchange pattern space with hold space
//d                           # if line matches PATTERN delete it
}' infile

or, in one line

sed '/PATTERN/!{h;d;};//{x;//d;}' infile

If you don't want to ignore consecutive matches:

sed '/PATTERN/!{              # if line doesn't match PATTERN
h                             # copy pattern space content over the hold buffer
d                             # delete pattern space
}
//x                           # if line matches PATTERN exchange buffers
' infile 

or, in one line

sed '/PATTERN/!{h;d;};//x' infile

Though keep in mind the solutions that use the hold buffer will print a leading empty line if the first line in your file matches.

In awk (note that this one will print any empty lines that come before the first non-empty one):

$ awk '{ if(! NF){$0=last}else{last=$0;}}1' file
101 abc group1
765 efg group2 
345 hij group4 
345 hij group4 
456 gfd group9 
762 ert group7 
762 ert group7 
762 ert group7 
554 fgt group11  

Explanation:

NF holds the number of fields. If the line is empty, there are no fields so the variable will be 0.

• if(! NF){$0=last}: if the number of fields is 0 (empty line), set the current line ($0) to the value of the variable last.
• else{last=$0;}: if there are fields, so this line is not empty, set last to hold the contents of this line.
• 1: the lone one at the end is an awk trick: when something evaluates to true (1 or any other integer greater than 0 is always true, since 0 is false) awk will print the current line. So that 1 is equivalent to print $0.

$ awk '! NF ? $0=last : last=$0;' file
101 abc group1
765 efg group2 
345 hij group4 
345 hij group4 
456 gfd group9 
762 ert group7 
762 ert group7 
762 ert group7 
554 fgt group11  

Explanation This is the same idea as above, but written in a more concise way. We are using the ternary operator. Since one of the two conditions will always be true (either NF is true or it is not true, so the ternary operator will always return true), both outcomes result in the line being printed (except for cases where the line is empty and no non-empty lines have been seen or if a line consistes of nothing but 0). However, if NF is not set, we set $0 to last and if it is set, we set last to $0. The result is the output we want.

Since the above will not print lines that are just 0, you can use this instead of that is a problem for you:

awk '{! NF ? $0=last : last=$0};1' file