snippet.ps1

$options = @{
    AddToHistoryHandler = {
        param( [string]$line)
        return $line.Length -gt 3 -and $line[0] -ne ' ' -and $line[0] -ne ';'
    }
}


Set-PSReadLineOption @options

Here's an example workaround using ILAssembler:

$handler = il ([Func[string, object]]) {
    ldarg.0
    brfalse.s fail_test

    ldarg.0
    callvirt { [int] [string].get_Length() }
    ldc.i4.3
    ble.s fail_test

    ldarg.0
    ldc.i4.0
    callvirt { [char] [string].get_Chars([int]) }
    ldc.i4.s 32
    beq.s fail_test

    ldarg.0
    ldc.i4.0
    callvirt { [char] [string].get_Chars([int]) }
    ldc.i4.s 59
    ceq
    ldc.i4.0
    ceq
    box { [bool] }
    ret

fail_test:
    ldc.i4.0
    box { [bool] }
    ret
}

Set-PSReadLineOption -AddToHistoryHandler $handler

Add-Type version:

Add-Type -TypeDefinition @'
namespace ProfileUtility
{
    public class PSRLUtilities
    {
        public static object AddToHistoryHandler(string line)
        {
            return line != null && line.Length > 3 && line[0] != ' ' && line[0] != ';';
        }
    }
}
'@


$handler = [ProfileUtility.PSRLUtilities].
    GetMethod('AddToHistoryHandler').
    CreateDelegate([Func[string, object]])

Set-PSReadLineOption -AddToHistoryHandler $handler

Probably slower startup time than the ILAssembler method, but for sure easier to read.