How can I rename files by replacing underscores with hyphens?

Here's a modified version of your regex that should work: `{ $_.Name -replace '^(.{11})_', '$1-' }`. This should effectively replace the underscore while keeping the rest of your script in check. Also, don't forget to add the `-WhatIf` parameter at the end of your command to preview changes before applying them!

Here’s a full script that will rename the files for you. It recursively goes through all directories and renames files by replacing the underscore after the 7 digits with a hyphen:

```powershell
Get-ChildItem -Path "C:YourFolderPath" -Recurse -File | ForEach-Object {
$oldName = $_.Name
if ($oldName -match '^(.{4}d{7})_(.+)$') {
$newName = $oldName -replace '^(.{4}d{7})_', '$1-'
Rename-Item -Path $_.FullName -NewName $newName -WhatIf
Write-Host "Would rename: $oldName -> $newName"
}
}
```
Just replace `"C:YourFolderPath"` with your actual directory path. This script even has a safety feature with `-WhatIf` so you can preview changes before actually renaming.

If you're open to alternatives, consider using Bulk Rename Utility. It's a user-friendly GUI for renaming files in bulk, or you could try its command line version. They’re both free and pretty effective for managing file names!

It looks like your regex pattern is slightly off. Your file name has the underscore right after the seven digits, so you need to adjust your code. Try modifying your regex to look for the underscore after those 7 digits. You might want to test it with something like this: `'^(.{4}d{7})_'` for your matching pattern. That should do the trick!