Batch script to rename files beginning with date format yyyymmdd

I have this batch script, but it is not renaming the files at all.

I have a list of documents that have been named in this format: yyyymmdd_docname1.pdf
I try to rename all of the documents to docname1_yyyymmdd.pdf.

Example to rename:

20230101_fullbloodcount.pdf to fullbloodcount_20230101.pdf.
20230202_esr.pdf to esr_20230202.pdf

What is constant for all these documents is the date format that is appended in front of the document name.

The script is as per below. I would appreciate if you could look over the script and let me know what is wrong with it.

@echo off
setlocal enabledelayedexpansion
set "directory=C:\Users\user1\Downloads\Rename"
cd /d "%directory%"

for %%f in (??????_*.pdf) do (

    set "filename=%%~nf"
    set "extension=%%~xf"
 
    set "datepart=!filename:~0,8!"
    set "docpart=!filename:~9!"
 
    set "newname=!docpart!_!datepart!!extension!"
 
    ren "%%f" "!newname!"
)

echo Files have been renamed.
pause

Solution

Here's a basic example of one way to approach the task:

@Echo Off
SetLocal EnableExtensions DisableDelayedExpansion

Set "Directory=%UserProfile%\Downloads\Rename"

PushD "%Directory%" 2>NUL || GoTo :EOF

For /F "Delims=" %%G In ('Dir /A:-D /B "????????_*.pdf" 2^>NUL') Do (
    If /I "%%~xG" == ".pdf" (
        Set "_=%%~nG"
        Set "$="
        Set /A "$=%%~nG" 2>NUL
        If Defined $ (
            Set "#=%%G"
            SetLocal EnableDelayedExpansion
            If !$! GEq 19700101 (
                If !$! LEq 20251231 (
                    Ren "!#!" "!_:~9!!_:~8,1!!$!%%~xG"
                )
            )
            EndLocal
        )
    )
)

PopD
EndLocal
Exit /B

I have added some simple date validation to this code, (instead of relying upon up to any eight characters, or even a sequence of eight digits). The above is designed to work with all valid dates from 1970 up until the end of this year, (you would need to adjust the LEq string beyond that). It does not however exclude invalid dates between those two, (e.g.20150229 or 20240931), as doing so would be well beyond the requirements of your stated problem, and simple batch scripting.