since this got answers, for full clarity:
Remove -WhatIf when you’re running it.
Get-ChildItem | Rename-Item -NewName {$_.Name -creplace "[A-Z]"} -WhatIf
This pipes Get-ChildItem into Rename-Item and then Sets the NewName to be the old name, except we’ve case-sensitively replaced any capital letters with nothing.
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solved Powershell & Regex – How to find and delete uppercase characters from file names in directory [closed]