How do I get rid of "--" line separator when using grep with context lines?

Question

I have a text file named compare.txt where I want to extract the single line that follows every line that contains the pattern nmse_gain_constant. The following command gets me close:

grep -A 1 nmse_gain_constant compare.txt | grep -v nmse_gain_constant 

But this includes a separator -- line between every line of desired text. Any easy ideas how to get rid of the -- lines?

Example: for an input file that looks like

line line nmse_gain_constant matching line line after first match line line nmse_gain_constant another matching line line after second match line nmse_gain_constant a third matching line line after third match 

the output is

line after first match -- line after second match -- line after third match 

but I'd like to have just

line after first match line after second match line after third match 

Answer 1

I do this:

 grep ... | grep -v -- "^--$" 

But this works too (on many, not all OS'es)!

grep --no-group-separator ... 

And it doesn't spit out that "--" or even a blank line.

Answer 2

There is an undocumented parameter of grep: "--group-separator", which overrides the default "--". You can set it to "" to get rid of the double dash. Though, you still get an empty line. I had the same trouble, and found this param by reading the source code of grep.