Batch .BAT script to rename files - regex

Question

Im looking for a batch script to (recursively) rename a folder of files..
Example of the rename:
34354563.randomname_newname.png to newname.png
I already dug up the RegEx for matching from the beginning of the string to the first underscore (its ^(.*?)_ ), but cant convince Windows Batch to let me copy or rename using RegEx.

Answer 1

From command-line prompt - without regex:
FOR /R %f IN (*.*) DO FOR /F "DELIMS=_ TOKENS=1,*" %m IN ("%~nxf") DO #IF NOT "%n" == "" REN "%f" "%n"
In batch file, double %:
FOR /R %%f IN (*.*) DO FOR /F "DELIMS=_ TOKENS=1,*" %%m IN ("%%~nxf") DO #IF NOT "%%n" == "" REN "%%f" "%%n"
EDIT: A new pure batch solution issuing following cases:
Path\File_name.ext => name.ext
Path\none.ext (does nothing)
Path\Some_file_name.ext => file_name.ext
Path\name.some_ext (does nothing)
Path\Some_file_name.some_ext => name.some_ext
Batch (remove ECHO to make it functional):
FOR /R %%f IN (*.*) DO CALL :UseLast "%%~f" "%%~nf"
GOTO :EOF
:UseLast
FOR /F "DELIMS=_ TOKENS=1,*" %%m IN (%2) DO IF "%%n"=="" (
IF NOT "%~2"=="%~n1" ECHO REN %1 "%~2%~x1"
) ELSE CALL :UseLast %1 "%%n"
GOTO :EOF

Answer 2

avoid _ in the extension:
#ECHO OFF
FOR /F "DELIMS=" %%A IN ('DIR /S /B /A-D *_*.*') DO FOR /F "TOKENS=1*DELIMS=_" %%B IN ("%%~NA") DO IF "%%~C" NEQ "" ECHO REN "%%~A" "%%~C%%~XA"
Look at the output and remove ECHO if it looks good.

Answer 3

The standard windows shell doesn't have regex capabilities, and in fact it's extremely basic. But you can use powershell to do this.
I'm not very familiar with power shell, but this other question explains how to filter files: Using powershell to find files that match two seperate regular expressions

Answer 4

New Solution
There is an extremely simple solution using JREN.BAT, my new hybrid JScript/batch command line utility for renaming files and folders via regular expression search and replace.
Only rename files with one underscore:
jren "^[^_]+_([^_]+\.png)$" "$1" /s /i
Preserve everything after the first underscore:
jren "^.*?_" "" /s /fm "*.png"
Preserve everything after the last underscore:
jren "^.*_" "" /s /fm "*.png"
=========================================================
Original Answer
This can be done using a hybrid JScript/batch utility called REPL.BAT that performs regex search and replace on stdin and writes the result to stdout. All the solutions below assume REPL.BAT is somewhere within your PATH.
I intentionally put ECHO ON so that you get a log of the executed rename commands. This is especially important if you get a name collision: two different starting names could both collapse to the same new name. Only the first rename will succeed - the second will fail with an error message.
I have three solutions that differ only in how they handle names that contain more than 1 _ character.
This solution will only rename files that have exactly one _ in the name (disregarding extension). A name like a_b_c_d.txt would be ignored.
#echo on&#for /f "tokens=1,2 delims=*" %%A in (
'2^>nul dir /b /s /a-d *_* ^| repl ".*\\[^_\\]*_([^_\\]*\.[^.\\]*)$" "$&*$1" a'
) do ren "%%A" "%%B"
The next solution will preserve the name after the last _. A name like a_b_c_d.txt would become d.txt
#echo on&#for /f "tokens=1,2 delims=*" %%A in (
'2^>nul dir /b /s /a-d *_* ^| repl ".*_([^\\_.]*\.[^.\\]*)$" "$&*$1" a'
) do echo ren "%%A" "%%B"
This last solution will preserve the name after the first _. A name like a_b_c_d.txt would become b_c_d.txt. This should give the same result as the Endoro answer.
#echo on&#for /f "tokens=1,2 delims=*" %%A in (
'2^>nul dir /b /s /a-d *_* ^| repl ".*\\[^\\]*?_([^\\]*\.[^.\\]*)$" "$&*$1" a'
) do echo ren "%%A" "%%B"