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I have a lot of video and text files. They all have the name: "Number-Suómething"

So for instance:

02-2.2-raKfcgvtavU.mp4

Is it possible to batch rename all the files using Mathematica to only "Number" ?

So for instance:

02.mp4

The following method does not require a specific fixed number of digits in the start of file name and does not depend on specific fixed non-numeric separator such as "-".

rename[name_]:=
RenameFile[
 name,
 StringCases[name,(x:DigitCharacter..)~~___~~".mp4":>x<>".mp4"][[1]]
]

1) Back up your files, - better make a new working copy of your directory.

2) Point Wolfram Language to that directory:

SetDirectory["path to your directory"]

3) Rename files:

rename /@ FileNames

You can also use CopyFile instead of RenameFile in order to keep the original file and place the renamed file in a directory of your choice. Because CopyFile can copy and rename simultaneously.

Simple
Just set the current working directory to the folder which contains the files:

SetDirectory["C\…"]

And then execute this command:

RenameFile[#, StringTake[ToString[#] , 2] <> ".mp4"] & /@ 
 FileNames["*.mp4"]

For .txt just do the same but replace the ".mp4" with ".txt"

Copy-Paste Code:

 SetDirectory["C\…"]
 RenameFile[#, StringTake[ToString[#] , 2] <> ".mp4"] & /@ 
 FileNames["*.mp4"]
 RenameFile[#, StringTake[ToString[#] , 2] <> ".txt"] & /@ 
 FileNames["*.txt"]

You can try to map the following function over a list of full file names (with their full path). Use at your own risk.

rename = file [Function] RenameFile[
 file,
 FileNameJoin[
 
 DirectoryName[file],
 StringSplit[FileBaseName[file], "-"][[1]] <> "." <> FileExtension[file]
 
 ]
 ]

For example, you can find all files with extension "*.mp4" in a given path path with

FileNames[FileNameJoin[path, "*.mp4"]]