Allow $null command prefixes

CHANGELOG.md

@@ -11,6 +11,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Fixed
 
 - Strip extra newlines (\n) from messages received from Teams backend.
+- Allow a `$null` command prefix. This is useful in backends like Teams that require you to @mention the bot (e.g. `@poshbot !status`).
+  This change removes the need for a command prefix as you are already specifying a bot command by @mentioning it.
+  The following syntax is now possible: `@poshbot status`
 
 ## [0.11.5] 2019-02-27
 

PoshBot/Classes/Bot.ps1

@@ -299,7 +299,14 @@ class Bot : BaseLogger {
      [bool]IsBotCommand([Message]$Message) {
         $firstWord = ($Message.Text -split ' ')[0].Trim()
         foreach ($prefix in $this._PossibleCommandPrefixes ) {
-            $prefix = [regex]::Escape($prefix)
+            # If we've elected for a $null prefix, don't escape it
+            # as [regex]::Escape() converts null chars into a space (' ')
+            if ([char]$null -eq $prefix) {
+                $prefix = ''
+            } else {
+                $prefix = [regex]::Escape($prefix)
+            }
+
             if ($firstWord -match "^$prefix") {
                 $this.LogDebug('Message is a bot command')
                 return $true