PowerShell and Regex to reformat dates in my filenames

Question

I am very new to Powershell. Bear with me.

I have a big directory of markdown files. They are either of the format <name>MMDDYY.md or <name>MMDDYYYY.md I'd like to use Powershell to rename them all to be in MMDDYY format.

Because my files are all from the year 2024, this is actually quite simple:

Get-ChildItem -recurse . | Rename-Item -NewName {$_.Name -replace '2024','24'}

But obviously I can't run this once I have more diverse filenames. I'd like a more general solution which automatically determine which files use MMDDYYYY format, and then rename only those ones. I know I could use a regex like \D*\d{2}\d{2}\d{4}.[m][d], but I'm not sure how to incorporate that into my Powershell command.

How could I do this?

Answer 1

Filter for files matching the pattern <name>MMDDYYYY.md with Where-Object:

Get-ChildItem -Filter *.md |
    Where-Object Name -Match '[0-9]{8}\.md$' |
    Rename-Item -NewName { $_.Name.Replace('2024','24') }

If you want something that could handle years dynamically, you can use a more complex pattern:

$files = @(
    'foobar10102024.md'
    'foobar10102023.md'
)
$files -replace '(?<=[0-9]{4})[0-9]{2}(?=[0-9]{2}\.md$)'

# foobar101024.md
# foobar101023.md

In practice:

Get-ChildItem -Filter *.md |
    Where-Object Name -Match '[0-9]{8}\.md$' |
    Rename-Item -NewName { $_.Name -replace '(?<=[0-9]{4})[0-9]{2}(?=[0-9]{2}\.md$)' }