Check inputs for bad characters, NaN, length - powershell

Question

I am searching for a general input check and I already found some things like NaN or if it is to short but how can you check if there are characters like ($§?=) which you do not want in your input to be?
My second question is what is important to check if you wanna rename a file using an input. Except length.
EDIT:
if($ParID -lt 6) {
$specific_error = "Par ID is is too short!"
} else { #else 1
if(!($ParID -match "[1-999999]")) {
$specific_error = "Par ID must only contain numbers!"
} else { #else 2
}#else 2
} #else 1
EDIT 2:
$ParIDInbox = New-Object System.Windows.Forms.TextBox #initialization -> initializes the input box
$ParIDInbox.Location = New-Object System.Drawing.Size(10,30) #Location -> where the label is located in the window
$ParIDInbox.Size = New-Object System.Drawing.Size(260,20) #Size -> defines the size of the inputbox
$ParIDInbox.MaxLength = 6 #sets max. length of the input box to 6

Answer 1

To check whether the filename contains any unwanted character you can use a regex:
$invalidCharacter = '$§?='
[regex]$invalidCharacter = '[{0}]' -f ([regex]::Escape($invalidCharacter))
if ($invalidCharacter.IsMatch($yourFileName))
{
# filename contains some invalid character ...
}
To ensure the filename is valid, you could use the GetInvalidFileNameChars .NET method to retrieve all invalid character and again use a regex to check whether the filename is valid:
[regex]$containsInvalidCharacter = '[{0}]' -f ([regex]::Escape([System.IO.Path]::GetInvalidFileNameChars()))
$yourFileName = 'invali?idFilename.txt'
if ($containsInvalidCharacter.IsMatch($yourFileName))
{
# filename is invalid...
}