A few comments 
escbackslash vs. escregexp: escbackslash is probably only useful outside renames (though it often will work). For any regex rename, use escregexp.
the modifier only affects what is inside the curly brackets. Apparently not
Rule: The modifier only affects what is inside the curly brackets!
Look further down for an exception 
single vs. double quotes: a pure JScript thing. I like to use single quotes for JScript and reserve the double quotes for Opus commands. Going "all double quotes" is possible, but then you would need even more \
The string a written within the script (I call this the "in script" form) has 4 backslashes see Note 2 below. Why?
The script variant has \\\\ because the normal command has \\. Why? Again, a pure JScript thing: "backslashes need to be escaped (with backslashes)". Period
So the more interesting question is: Why does the normal variant use \\?
Well, I thought I had a good explanation. Turned out I didn't. At least not in the way I originally envisioned. I now think it's a bug. I've written a separate report.
General advice: There is nothing wrong with using codes in Rename statements, especially if you want to be able to later/again/also use the statements in normal (non-script) buttons. But if you go all-in with scripting, use the item properties and the JScript regex support. It'll make your code a lot easier to understand/debug/maintain 