Validate that a PowerShell parameter is alphanumeric

Question

I simply want to validate that a parameter is only made up of alphanumeric characters. I've tried this, but it doesn't behave as I'd like:

param(
    [ValidatePattern('[a-zA-Z0-9]')]$someVariableThatShouldOnlyContainAlphaNumerics = 'something_with_underscores'
)
write-host $someVariableThatShouldOnlyContainAlphaNumerics

Returns:

something_with_underscores

I'm clearly missing something obvious. Any suggestions?

Answer 1

ValidatePattern supports regex. If the pattern matches the string is validated. You are not validating all characters in the string... just one !

Changing the regex pattern to match the entire string would be one approach. Other regex will also suffice.

[ValidatePattern('^[a-zA-Z0-9]+$')]$someVariableThatShouldOnlyContainAlphaNumerics = 'something_with_underscores'

Note that setting a default will bypass this validation as it is not being passed to the function.