if (Test-Path $path) {
    Write-Host "File exists."
}
if (!(Test-Path $path)) {
    Write-Host "File does not exist."
}

Get-ChildItem -Recurse -Path C:\Temp -Include *.txt | Select-String 'This', 'OR that' | Select-String 'AND this' | Format-Table -property path, line -autosize | Out-String -Width 4096

(Get-Content $filename) -replace 'heise', 'Heise' | Set-Content $filename

param (
    [parameter(Mandatory=$true)] [string]$targetServer,
    [parameter(Mandatory=$true)] [string]$targetWebConfig
)

Get-ChildItem -Path Cert: -Recurse | Where-Object { $_.Thumbprint -and $_.Thumbprint.ToLower().StartsWith('c4') }

'1234abcd' -replace '\d', '*' # ****abcd

'123.456.789.123' -match '\d+(?:\.\d+){2}' # returns true if there is a match.
$Matches.Values                            # = '123.456.789'

$inputText = '!! Match me
Not me
!! Match me too
Not me tho'
$regex=[regex] '(?m)^!!.+?$'
$regex.matches($inputText) | Foreach-Object { $_.Value } # Returns a string, one match per line.

# Filter lines by regex.
$inputText = ' .. . . left:10 .  .. right:10 ... 
 .. . . left:20 .  .. right:20 ... 
 .. . left:30 . . ...  .. right:30 ... 
 left:40right:40' -replace "`r`n", "`n"
 
$regex=[regex] '(?m)^[ \.]*(?<left>left:\d+)[ \.]*(?<right>right:\d+)[ \.]*$'
$regex.matches($inputText) | Foreach-Object { $_.Groups["left"].value + ' - ' + $_.Groups["right"].value}
# Returns a string:
#left:10 - right:10
#left:20 - right:20
#left:30 - right:30
#left:40 - right:40

# $1 = group 1, $2 = group 2, etc.
'123 456 789' -replace '(\d+ )(\d+)( \d+)', '$1!$2!$3'  # = '123 !456! 789'