Optimizing a script

I would trade this:

$dnslog = [System.IO.File]::Open("c:\dns.log","Open","Read","ReadWrite")
$dnslog_content = New-Object System.IO.StreamReader($dnslog)


For ($i=0;$i -lt $dnslog.length; $i++)
{


    $line = $dnslog_content.readline()
    if ($line -eq $null) { continue }


    # REGEX MATCH EACH LINE OF LOGFILE
    $pattern = $line | select-string -pattern $regex



    # IGNORE EMPTY MATCH
    if ($pattern -eq $null) {
            continue
    }

for this:

Get-Content 'c:\dns.log' -ReadCount 1000 |
 ForEach-Object {
   foreach ($line in $_)
    {
      if ($line -match $regex)
       {
         #Process matches
       }
    }

That will reduce then number of file read operations by a factor of 1000.

Trading the select-string operation will require re-factoring the rest of the code to work with $matches[n] instead of $pattern.matches[0].groups[$n].value, but is much faster. Select-String returns matchinfo objects which contain a lot of additional information about the match (line number, filename, etc.) which is great if you need it. If all you need is strings from the captures then it's wasted effort.

You're creating an object ($log), and then accumulating values into array properties:

$log.date                += @($pattern.matches[0].groups[$n].value); $n++

that array addition is going to kill your performance. Also, hash table operations are faster than object property updates.

I'd create $log as a hash table first, and the key values as array lists:

$log = @{}
$log.date = New-Object collections.arraylist

Then inside your loop:

$log.date.add($matches[1]) > $nul)

Then create your object from $log after you've populated all of the array lists.