PowerShell script to convert a WebVTT caption file (*.vtt) to SubRip (*.srt)

convert-vtt.ps1

# SRT File format https://docs.fileformat.com/video/srt/
# WebVTT file format https://developer.mozilla.org/en-US/docs/Web/API/WebVTT_API
param (
    [Parameter(Mandatory = $true)][string]$localPath
    )

# Get all of the matching file names
$MatchingFileNames = (Get-ChildItem $localPath -File) | sort-Object Name

# walk through the list of files
foreach($MatchingFileName in $MatchingFileNames)
{
    $file_data = Get-Content $MatchingFileName

    # Verify that we are reading a WebVTT file
    $IsVTT = $file_data[0] -like 'WEBVTT*'

    if ($IsVTT -eq $True)
    {
        # Generate the destination filename
        $srtFilename = [io.path]::ChangeExtension($MatchingFileName.FullName, 'srt')
        $new_data = New-Object System.Collections.Generic.List[System.Object]

        Write-Host ('Reading ' + $MatchingFileName.FullName )
        $CurentLine = 2
        $Counter = 1

        while ($CurentLine -lt $file_data.Length)
        {
            $ThisLine = $file_data[$CurentLine]

            $ThisTime = $ThisLine.Split(" ")

            if ($ThisTime.Length -eq 3)
            {
                if ($ThisTime[1] -eq "-->")
                {
                    $ThisLine = $ThisLine.Replace('.', ',')

                    $new_data.Add($Counter++)
                    $new_data.Add($ThisLine)

                    $CurentLine++

                    # Read the next set of lines for the caption
                    # Treat as caption until the next blank line

                    $caption = $file_data[$CurentLine]
                    while ($caption -ne '')
                    {
                        $new_data.Add($caption)
                        $caption = $file_data[++$CurentLine]
                    }

                    $new_data.Add($caption)
                }
            }
            $CurentLine++
        }

        # Write out the converted data
        Write-Host ('Writinging ' + $srtFilename )
        Out-File -FilePath $srtFilename -InputObject $new_data
    }
}