Actually, it is only the RHS that the user is passing. I mislead you because of my example. I only used that because of my curiosity as to why it worked on one side but not the other.

The pattern to be replaced is provided by the script, ie, the script searches for illegal character sequences and interactively replaces them with the user's input.

What I came up with by the direction of others here seems to work fine. I'm happy so far.