PowerShell: Batch Renumber Files

The script

# Shift every file whose name starts with a number >= 15 up by one.
# Run from the folder containing the files.
$from = 15   # first number to shift

Get-ChildItem -File |
  Where-Object { $_.Name -match '^(\d+)' -and [int]$Matches[1] -ge $from } |
  Sort-Object { [int][regex]::Match($_.Name, '^\d+').Value } -Descending |
  ForEach-Object {
    $numText = [regex]::Match($_.Name, '^\d+').Value
    $newNum  = ([int]$numText + 1).ToString("D$($numText.Length)")
    $newName = $_.Name -replace '^\d+', $newNum
    Rename-Item -LiteralPath $_.FullName -NewName $newName
  }

To preview the renames without executing them, add -WhatIf to the Rename-Item line, run it once, and read the output. Remove it when you're satisfied.

The three traps it avoids

• Rename collisions. Processing in ascending order renames 15 → 16 while a real file 16 still exists. Sort-Object … -Descending renames from the top down (50 → 51 first), so a target name is always free.
• Lost zero-padding. 09 + 1 naively becomes 10, but 099 must become 100 and 05 must become 06, not 6. The script measures each filename's own digit count and re-pads with ToString("D<width>").
• Unnumbered files. The Where-Object filter skips anything that doesn't start with digits, so a stray notes.txt in the folder is untouched.

What the script can't see

It assumes the number is at the start of the filename and that your set is consistent. Mixed conventions — 1. Contract.pdf next to 01_Contract.pdf, or an "Exhibit 15" prefix — need the regex adjusted, and duplicate numbers in the source folder will silently produce wrong results.

If you'd rather not maintain the script, Renumber Files does the same job from a pasted file list: it auto-detects the numbering style, warns about duplicates, previews every rename, and handles multiple insertion points in one pass — locally in your browser, on Windows and macOS.