Extract IP address from .txt file using Powershell and save it into a new text file

I have a text file and it's contains an IP address what I want to extract and save it to a new text file. The text file that contains the IP address it's an output file from an application called ngrok. That's how the file looks like:

t=2023-07-05T16:17:15+0300 lvl=info msg="no configuration paths supplied"
t=2023-07-05T16:17:15+0300 lvl=info msg="using configuration at default config path" path=C:\\Users\\***\\AppData\\Local/ngrok/ngrok.yml
t=2023-07-05T16:17:15+0300 lvl=info msg="open config file" path=C:\\Users\\***\\AppData\\Local\\ngrok\\ngrok.yml err=nil
t=2023-07-05T16:17:15+0300 lvl=info msg="starting web service" obj=web addr=127.0.0.1:4040 allow_hosts=[]
t=2023-07-05T16:17:15+0300 lvl=info msg="tunnel session started" obj=tunnels.session
t=2023-07-05T16:17:15+0300 lvl=info msg="client session established" obj=csess
t=2023-07-05T16:17:15+0300 lvl=info msg="started tunnel" obj=tunnels name=command_line addr=//localhost:8865 url=tcp://4.tcp.eu.ngrok.io:10355
t=2023-07-05T16:17:16+0300 lvl=info msg="update available" obj=updater

The address that I want to extract is 4.tcp.eu.ngrok.io:10355

Recently I created this powershell script:

$List_IP = Get-ChildItem -Path "C:\ngrok_log.txt"

foreach ($IP in $List_IP)
{
	$pattern = '(\d.tcp.eu.ngrok.io\W\d\d\d\d\d)'

	$sel = select-string -list $IP -pattern $pattern | Select-Object -ExpandProperty Line
	If ($Sel -eq $null)
{
	write-host "ERROR - $IP does not contain $pattern"
}
Else
{
	write-host "SUCCESS - $pattern `n$sel"
}

Write-host "end"
	$Folder = "C:\"
	$SaveFile = $Folder + "final_ip.txt"
	$sel | Out-File $SaveFile
}

It works, but it's saving the whole line that contains the IP address instead just the IP address:

t=2023-07-05T16:17:15+0300 lvl=info msg="started tunnel" obj=tunnels name=command_line addr=//localhost:8865 url=tcp://4.tcp.eu.ngrok.io:10355

I don't really understand the "pattern" part of this script, everytime I restart the ngrok I get new random IP address like this:

(number between 1-99).tcp.(ap, au, eu, in, jp, sa, us).ngrok.io:(number between 10000-99999)

So.., how can I extract the IP address from that file and save it to a new one?

Your code looks good, the main issue is when you do "...$sel" you're coercing a MatchInfo instance to be a string and when coerced the result is its .Line property when what you're actually looking for is the matched value of your pattern, the same would be happening in $sel | Out-File ....

A simple example:

$match = 'foo bar baz' | Select-String bar
$match.Line          # foo bar baz
$match.ToString()    # foo bar baz
$match.Matches.Value # bar

So those 2 lines should be changed to:

Write-Host "SUCCESS - $pattern `n$($sel.Matches.Value)"

# and
$sel.Matches.Value | Out-File ...

This line also seem to be not needed:

$List_IP = Get-ChildItem -Path "C:\ngrok_log.txt"

And | Select-Object -ExpandProperty Line should definitely be removed.

As for the regex pattern, what you have currently might work fine but if you want more precision you can give this one a try:

(?<=url=tcp://)[1-9][0-9]?\.tcp\.(?:ap|au|eu|in|jp|sa|us)\.ngrok\.io:[1-9][0-9]{4}\b

See https://regex101.com/r/abddxD/1 for details.

Summarizing, you can simplify your code as follows:

$pattern = '(?<=url=tcp://)[1-9][0-9]?\.tcp\.(?:ap|au|eu|in|jp|sa|us)\.ngrok\.io:[1-9][0-9]{4}\b'
$match = Select-String -LiteralPath path\to\ngrok_log.txt -Pattern $pattern

if ($match) {
    Write-Host "SUCCESS - $pattern `n$($match.Matches.Value)"
    $folder = 'C:\'
    $saveFile = Join-Path $Folder -ChildPath 'final_ip.txt'
    $match.Matches.Value | Out-File $SaveFile
}
else {
    Write-Host "ERROR - $IP does not contain $pattern"
}

Write-Host 'end'