How to mask and replace characters in a string, based on pattern?

Question

test contains string, which checks for a particular pattern. If pattern doesn't exist, find at least a character of pattern and it's position and re-frame the test consisting of provided pattern.

1. In scenario 1, pattern exists. So, no replacement.

test = "aebfz"
pattern = "ebf"
output = No replacement required

2. In scenario 2, no pattern exists but 'b' exists. After re-framing, out put should look like below (position of 'b' in test is 1. 'b' in pattern is '1')

> test = "abcdz" 
> pattern = "ebf"        
> output = "ebfdz"

3. In Scenario 3, no pattern exists. But both 'b' and 'e' exists at different positions. So, possible output looks like below ('b' is '0' and 'e' is '4' in test)

>  test = "bacde"
>  pattern = "ebf"
>  output1 = "ebfcde" (added 'e' before 'b')
>  output2 = "bacdebf" (added 'bf' after 'e')

4. In scenario 4, no pattern exists. But char 'b' exists, so as output.

test = "xabdz"   
pattern = "ebf"    
Output = "xebfz"

5. In scenario 5, no pattern exists. Output can be re-framed as below

test = "vwxyz"
pattern = "ebf"
Output1 = "ebfyz"
Output2 = "vebfz"
Output3 = "vwebf"

I'm thinking to mask and replace mechanism. For example: scenario 2

test = "abcdz" 
pattern = "ebf" 
new_test = "#b#dz" (should look like)
final_test = "ebfdz"

I don't know how it can be done. Just my approach. Can you please help me here ? If my approach is not right, please let me know possible best option.