shell - How to use sed to get matched pattern and replace another line text

Question

I want use sed to replace a pattern.

1. Go through each row
2. Get each title, left of ":" (Line1) save its content "Line1_content"
3. Check the "to be replaced part" on the right side of ":" in each row. find matched pattern "Line1" , and then replace it with saved content "Line1_content".

I prefer to use sed to save pattern and swap it to the matched part.

Example input:

Line1: content_A
Line2: content_B 
Line3: Line1
Line4: nonsense & Line2

Example output:

Line1: content_A
Line2: content_B 
Line3: content_A
Line4: nonsense & content_B

Answer 1

等待大神答复