sed replace a word at a line which begins with a specific pattern using

Question

How can I replace a word at a line which begins with a specific pattern on FreeBSD? Consider the following file contents:

this is to test 
that was for test

I want to replace "test" at the line which begins with "this".

Answer 1

In order to perform a replacement for lines starting with this, say:

$ sed '/^this/ s/test/something/' inputfile
this is to something 
that was for test

This would replace the word test with something on lines starting with this.

If you want to replace all instances of test on the matching lines, supply the g option to sed:

sed '/^this/ s/test/something/g' inputfile