powershell user input validation not accepting valid input

Question

I have some code which is behaving strangely and I am not sure why. I am attempting to validate the user input is a number and that it is less than 255. Pretty easy.

Problem is that numbers from 26 to 99 are not valid for me in my testing. 1-25 are fine, and 100+ seem fine too.. but for some reason 26-99 keep me in the loop.

DO
{
$ip_addr_first = Read-Host 'enter a number less than 255'
} while ($ip_addr_first -notmatch '\p{Nd}+' -or $ip_addr_first -gt 255) 

write-host 'You entered' $ip_addr_first

suggestions welcome on where the problem is, as I'm at a loss here.

mklement0 · Accepted Answer · 2016-07-27 03:30:24Z

0

Try

do
{
  $ip_addr_first = Read-Host 'Enter an integer between 0 and 255'
} while ($ip_addr_first -notmatch '^\p{Nd}+$' -or [int] $ip_addr_first -gt 255) 

write-host 'You entered:' $ip_addr_first

Note the [int] cast, which ensures that $ip_addr_first is compared as a number, which makes all the difference here.

Without it, given that $ip_addr_first is a string (the type returned by Read-Host), -gt performs lexical comparison, and 26 is lexically greater than 255, for instance (the same applies to any number (string) starting with 3 or greater).

Also note that I've anchored your regular expression p{Nd}+ with ^ and $ to ensure that the entire input is matched against it (you could make this more permissive by allowing leading and trailing whitespace).

Alternative approach that uses number parsing only:

[int] $ip_addr_first = -1
do
{
  $userInput = Read-Host 'Enter an integer between 0 and 255'
} while (-not ([int]::TryParse($userInput, [ref] $ip_addr_first) -and
               $ip_addr_first -ge 0 -and $ip_addr_first -le 255)) 

write-host 'You entered:' $ip_addr_first

edited Jul 27, 2016 at 3:30

answered Jul 27, 2016 at 3:07

mklement0

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2 Comments

PS_newbie Over a year ago

thanks... the cast is a new one for me since I'm just learning Powershell

mklement0 Over a year ago

@PS_newbie: My pleasure; in PowerShell you often don't have to worry about types, but on occasion you do - generally, it's worth understanding the underlying (.NET) types and how they can be inspected (Get-Member) and converted. My alternative solution shows you how to type-constrain a variable during initialization (a persistent cast, in a manner of speaking).

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