Regular expression convert Date format from dd-mon-yy to dd/MM/yy

You should use [DateTime]::ParseExact() as regex would require 12 different replace-operations, or a MatchEvalutor to convert the month.

Example using regex MatchEvaluator:

$MatchEvaluator = {  
  param($match) 

  #Could have used a switch-statement too..
  $month = [datetime]::ParseExact($match.Groups[2].Value,"MMM",$null).Month

  "{0:00}/{1:00}/{2:00}" -f $match.Groups[1].Value, $month, $match.Groups[3].Value
}

[regex]::Replace("25-FEB-16","(\d+)-(\w+)-(\d+)", $MatchEvaluator)
25/02/16

Looking at that, I would say that using only ParseExact() is a much better solution:

try {
    [datetime]::ParseExact("25-FEB-16","dd-MMM-yy", $null).ToString("dd/MM/yy", [cultureinfo]::InvariantCulture)
} catch {
    #Invalid date-format
}
25/02/16