PowerShell: How do I append a prefix to all files in a directory?

An alternative approach to a regular expression would be to use simple string operations to create the new file name.

$items=Get-ChildItem;
$items | Rename-Item -NewName { "Prefix_" + $_.Name };

You are quite near.

• -replace uses RegEX and in a Regular Expression you anchor at the beginning with a ^
• for a suffix you can similarly replace the anchor at line end with $
• to avoid any possible re-iteration of the renamed file enclose the first element of the pipeline in parentheses.

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(Get-ChildItem -File) | Rename-Item -NewName {$_.Name -replace "^","Prefix_"}