Comparison of strings using variables always false (in Batch)

Question

In my current folder, I have a file named backup(yyyy-mm-dd).7z
I need to keep that length variable even though here it is a constant.

@echo off
setlocal EnableDelayedExpansion

set "a=.\backup"
set "b=yyyy-mm-dd"
set "length=19"

for %%f in (.\*) do (

    set "fullpath=%%f"
    set "trimpath=!fullpath:~0,%length%!"

    set trimpath
    echo trimpath=%trimpath%

    if %trimpath% == %a%(%b% echo this is equal
)

I have 2 questions regarding that code:

1. Why can I see the value of trimpath when I call set trimpath but not directly with %trimpath%?
2. Why is my condition false? What should I do to get it true?

Answer 1

Here is the solution after following @foxidrive and @JosefZ advices!

@echo off
setlocal EnableDelayedExpansion

set "a=.\backup"
set "b=yyyy-mm-dd"
set "length=19"

for %%f in (.\*) do (

    set "fullpath=%%f"
    set "trimpath=!fullpath:~0,%length%!"

    echo trimpath=!trimpath!

    if "!trimpath!" == "%a%(%b%" echo this is equal
)

If you need to test the solution, just create a file named backup(yyyy-mm-dd) in your current folder.