Find line in file and replace it with line from another file - awk

Question

My goal is to find a string within a file (file1) and replace its whole line with the content of a specific line (in this example line 3) from another file (file2). As I understand, I need to use RegEx to do the first part and then use a second sed command to store the contents of file2. sed is definitely not my strong suit, so I hope someone here can help a rookie out!
So far I have:
sed -i '/^matching.string.here*/s' <(sed '3!d' file2) file1
Edit
Example file1:
string one
string two
matching.string.here.
string three
Example file2:
alt string one
alt string two
alt string three
Expected Result in file1:
string one
string two
alt string three
string three

Answer 1

Your sed attempt contains several unexplained errors; it's actually hard to see what you are in fact trying to do.
You probably want to do something along the lines of
sed '3!d;s%.*%s/^matching\.string\.here.*/&/%' file2 |
sed -f - -i file1
It's unclear what you hope for the /s to mean; does your sed have a flag with this name?
This creates a sed script from the third line of file2; take out the pipeline to sed -f - to see what the generated script looks like. (If your sed does not allow you to pass in a script on standard input, you will have to write it to a temporary file, and then pass that to the second sed.)
Anyway, this is probably both simpler and more robust using Awk.
awk 'NR==3 && NR==FNR { keep=$0; next }
/^matching\.string\.here/ { $0 = keep } 1' file2 file1
This writes the new content to standard output. If you have GNU Awk, you can explore its -i inplace option; otherwise, you will need to write the result to a file, then move it back to file1.

Answer 2

This might work for you (GNU sed):
sed -n '3s#.*#sed "/matching\\.string\\.here\\./c&" file1#ep' file2
Focus on line 3 of file2.
Manufacture a sed script which changes a matching line in file1 to the contents of the line in focus and print the result.
N.B. The periods in the match must be escaped twice so as not to match an arbitrary character.

Answer 3

This is tailor made job for awk and bonus is that you can avoid any regex:
awk -v s='matching.string.here' 'FNR == NR {a[FNR] = $0; next} index($0, s) {$0 = a[FNR]} 1' file2 file1
string one
string two
alt string three
string three
A more readable version:
awk -v s='matching.string.here' '
FNR == NR {
a[FNR] = $0
next
}
index($0, s) {
$0 = a[FNR]
} 1' file2 file1