Adding ValidateExtension (#64)

Author: James Brundage

Transpilers/Parameters/ValidateExtension.psx.ps1

@@ -0,0 +1,60 @@
+<#
+.SYNOPSIS
+    Validates Extensions
+.DESCRIPTION
+    Validates that a parameter or object has one or more extensions.
+    
+    This creates a [ValidatePattern] that will ensure the extension matches.
+.Example
+    {        
+        param(
+        [ValidateExtension(Extension=".cs", ".ps1")]
+        $FilePath
+        )
+    } |.>PipeScript
+.Example
+    .> {
+        param(
+        [ValidateExtension(Extension=".cs", ".ps1")]
+        $FilePath
+        )
+
+        $FilePath
+    } -Parameter @{FilePath=".ps1"}
+.EXAMPLE
+    .> {
+        param(
+        [ValidateExtension(Extension=".cs", ".ps1")]
+        $FilePath
+        )
+
+        $FilePath
+    } -Parameter @{FilePath="foo.txt"}
+#>
+[CmdletBinding(DefaultParameterSetName='Parameter')]
+param(
+# The extensions being validated.
+[Parameter(Mandatory,Position=0)]
+[string[]]
+$Extension,
+
+# A variable expression.
+# If this is provided, will apply a ```[ValidatePattern({})]``` attribute to the variable, constraining future values.
+[Parameter(ValueFromPipeline,ParameterSetName='VariableExpressionAST')]
+[Management.Automation.Language.VariableExpressionAST]
+$VariableAST
+)
+
+process {
+    $validExtensionRegex = "\.(?>$($extension -replace '^\.' -join "|"))$"
+[ScriptBlock]::Create(@"
+[ValidatePattern('$($validExtensionRegex.Replace("'","''"))')]
+$(
+    if ($psCmdlet.ParameterSetName -eq 'Parameter') {
+        'param()'
+    } else {
+        '$' + $VariableAST.variablePath.ToString()
+    }
+)
+"@)
+}