正则表达式IF条件

Question

I'm looking for a way to filter new IP-Addresses in my host file. I created a script which updates my host file every time when I call it with data from the matrix enterprise manager. It works fine. But I have to find a solution that only 10.XX.XX.XX or 172.XX.XX.XX addresses are allowed to update.

Param(
  $newHost = $args[0],
  $newIP = $args[1]
)

$SourceFile = "hosts"
$Match = "$newHost"

(Get-Content $SourceFile) | % {if ($_ -notmatch $Match) {$_}} | Set-Content $SourceFile

Start-Sleep -Seconds 1

$tab = [char]9
$enter = $newIP + $tab + $newHost

if ($newIP XXXX)  #--> here should be a regex if condition... no clue how it works..

$enter | Add-Content -Path hosts

Answer 1

Your code is unnecessarily convoluted and doesn't make proper use of the features PowerShell provides.

• Don't assign $args[...] to parameters. That's not how parameter handling in PowerShell works. Make the parameters mandatory instead.
• % {if ($_ -notmatch $Match) {$_}} is better phrased as Where-Object {$_ -notmatch $Match} .
• If $Match is an FQDN the dots might cause false positives (because they match any character, not just literal dots). Either escape $Match ( [regex]::Escape($Match) ) or use the -notlike operator instead.
• PowerShell has an escape sequence for tabs ( `t ). No need to define a variable with a value of [char]9 .
• Putting variables in a double-quoted string ( "$var1$var2" ) is often more readable than string concatenation ( $var1 + $var2 ).

Change your code to something like this:

[CmdletBinding()]
Param(
  [Parameter(Mandatory=$true)]
  [string]$Hostname,
  [Parameter(Mandatory=$true)]
  [string]$IPAddress
)

$SourceFile = 'hosts'

(Get-Content $SourceFile) |
  Where-Object { $_ -notlike "*$Hostname*" } |
  Set-Content $SourceFile

Start-Sleep -Seconds 1

if ($IPAddress -match '^(10|172)\.') {
  "$IPAddress`t$Hostname" | Add-Content $SourceFile
}

If you want to avoid writing the output file multiple times, you could collect the data read in a variable, and then write that variable and the new record in one go:

$hosts = @(Get-Content $SourceFile) | Where-Object { $_ -notlike "*$Hostname*" })

if ($IPAddress -match '^(10|172)\.') {
  $hosts += "$IPAddress`t$Hostname"
}

$hosts | Set-Content $SourceFile

You could further optimize your script by doing the check via parameter validation , so you don't need an if condition in the function body in the first place, eg like this:

Param(
  [Parameter(Mandatory=$true)]
  [string]$Hostname,
  [Parameter(Mandatory=$true)]
  [ValidatePattern('^(10|172)\.')]
  [string]$IPAddress
)

or like this:

Param(
  [Parameter(Mandatory=$true)]
  [string]$Hostname,
  [Parameter(Mandatory=$true)]
  [ValidateScript({$_ -match '^(10|172)\.' -and [bool][ipaddress]$_})]
  [string]$IPAddress
)

Answer 2

Some of the comments above use regex to validate the entire IPv4 address. If you are confident that the IP address you are checking is valid, then you can use "^(10|172)\\." to verify just the first octet of the address as per your question:

if($newIP -match "^(10|172)\.") { ... }

If you do want to validate the whole address, there is an alternative way to do this by converting $newIP to a [System.Net.IPAddress] type. If this fails the result will be nothing (null), which is implicitly false, so the following gives you a true/false check that a string is a valid IP address:

[bool]($newIP -as [System.Net.IPAddress])

You could use this to validate the input to the function written by Ansgar:

[Parameter(Mandatory=$true)]
[ValidateScript({[bool]($_ -as [System.Net.IPAddress]})
[string]$IPAddress