VSCode: Snippet - Transform characters to another equal length line

What I would like: A snippet, call that Bound Line. When I trigger that snippet and type something like:

This is my line

and hit tab, it should print:

This is my line
    C
***************

'C' where the cursor should be and "****" line is exactly same number of characters as the first line. After looking over the BNF for transformations at Microsoft Documentation, I came up with the following which is horribly, horribly wrong:

"Bound Line" : {
        "prefix" : "bl",
        "body": [
            "${2:}",
            "\t$3""${2/(.*)/${1:/[a-zA-Z0-9:,;*_]/*/gi}"
        ]
    }

If you have some insights, I would appreciate it. Thanks!