Verifying a string is not empty/null in an If condition - powershell

Question

I'm writing a script that will accept user input via Read-Host (set to $String), and I want to avoid any issues that could be caused by having a blank value for the variables. Since I'll be using this a lot, I want to implement it into a function that verifies no invalid characters are being used.
I thought I could use an if statement with ![string]::IsNullOrEmpty($String) as one of the conditions:
Function Test-ValidCharacters ($String, $ValidCharacters) {
if (($String -match $ValidCharacters) -and (!([string]::IsNullOrEmpty($String)))) {
return $true
}
else {return $false}
}
I also tried this:
Function Test-ValidCharacters ($String, $ValidCharacters) {
if (($String -match $ValidCharacters) -and ($String -ceq "")) {
return $true
}
else {return $false}
}
In both of these cases, I can just hit enter when presented with the $String's Read-Host prompt and the script will behave as if the function returned $True (and then later encounter fatal errors). The other half works - if I include characters not specified by $ValidCharacters the function returns $False as expected.
I am sure I'm missing something here. I even tried doing a second nested if statement and got the same result.
Edit: Here's the code snippet where I call the function and notice the issue.
$ValidCharacters = '[^a-zA-Z0-9]'
$FirstN = Read-Host -Prompt "New user's first name"
While (Test-ValidCharacters $FirstN $ValidCharacters -eq $false) {
Write-Output "$FirstN contains illegal characters. A-Z, a-z, and 0-9 are accepted."
$FirstN = Read-Host -Prompt "New user's first name"
}

Answer 1

Assuming $ValidCharacters isn't itself an empty string and contains an anchored character-range regex (regular expression) that covers the entire input string, such as ^[a-z0-9./:]+$, given that the -match operator matches any substring by default (note that a better name for the parameter is therefore something like $ValidationRegex):[1]
In the first function definition, the RHS of your -and operation is redundant - it adds nothing to the conditional, because if $String -match $ValidCharacters is $true, then so is ! [string]::IsNullOrEmpty($String), by definition.
Conversely, in the second function definition your -and operation always returns $false, because $String -ceq "" is by definition $false, if the LHS returned $true.
Assuming that your intent is to prevent empty or all-whitespace input and to ensure that any string - trimmed of incidental leading and/or trailing whitespace - is composed only of expected characters, use the following:
Function Test-ValidCharacters ($String, $ValidCharacters) {
# Note: no strict need for `return`.
$String.Trim() -match $ValidCharacters
}
[1] Alternatively, stick with $ValidCharacters and pass a regex that describes only a single valid character, such as '[a-z0-9./:]', and construct the entire-string matching regex inside the function with '^' + $ValidCharacters + '+$'