Powershell input validition - validation

Question

I am trying to only allow a user to enter 2 letters when they rename a file if its exists.
It is not working as I wanted it. I am allowed to enter numbers.
if ($RenameLargeFiles.Length -gt '2') {
Write-Host " You may only enter 2 letters."
Pause
Clear-Host
Rename-LargeFiles
} else {
Rename-Item -Path $LargeFiles $RenameLargeFiles".txt"
$LargeFiles = $RenameLargeFiles
Set-Content -Value $Files -Path $LargeFiles
}
}
Set-StrictMode –Version Latest
$LargeFiles = "C:\PSScripts\LargeFiles.txt"
$PSScripts = "C:\PSScripts"
$TestPSScripts = Test-Path "C:\PSScripts"
switch ($TestPSScripts) {
'False' { New-Item -Type directory -Path $PSScripts }
}
function Test-Files {
$Files8 = ""
$Files8 = Read-Host " Please Enter the Complete Path. "
$TFiles8 = Test-Path $files8
if ($TFiles8 -eq 'True') {
Write-Host $Files8 "LOCATED."
} else {
Write-Host $Files8 "NOT FOUND"
Pause
Clear-Host
Test-Files
}
}
function Test-LargeFiles {
$LargeFiles = "C:\PSScripts\LargeFiles.txt"
$TestLargeFiles = Test-Path $LargeFiles
if ($TestLargeFiles -eq 'True') {
Write-Host $LargeFiles "already present"
Rename-LargeFiles
} else {
Write-Host $LargeFiles "NOT FOUND."
Write-Host $LargeFiles "created"
}
}
function Rename-LargeFiles {
$LargeFiles = "C:\PSScripts\LargeFiles.txt"
[string] $RenameLargeFiles = Read-Host " Please Enter 2 letters to rename" $LargeFiles
if ($RenameLargeFiles.Length -gt '2') {
Write-Host " You may only enter 2 letters."
Pause
Clear-Host
Rename-LargeFiles
} else {
Rename-Item -Path $LargeFiles $RenameLargeFiles".txt"
$LargeFiles = $RenameLargeFiles
Set-Content -Value $Files -Path $LargeFiles
}
}
Test-Files
$Files = Get-ChildItem -Recurse $Files8 | Sort-Object -Descending -Property Length | Select-Object -First 10
Test-LargeFiles
Add-Content -Value $Files -Path $LargeFiles
pause

Answer 1

Use a regex like so:
if ($RenameLargeFiles -notmatch '[aA-zZ]{2}') {
... display error message
}