From 90b938489916060859702ad4f5eecb96deb9730f Mon Sep 17 00:00:00 2001
From: Jeremy Studer <studerj1.mail@gmail.com>
Date: Thu, 22 Mar 2018 23:34:22 -0400
Subject: [PATCH] Panic when given empty character class <[]>

Check whether the QAST node has any children to determine if it is
empty.  The exception are the backslash cclass nodes (\b, \n, \s, etc).
---
 src/QRegex/P6Regex/Actions.nqp | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/QRegex/P6Regex/Actions.nqp b/src/QRegex/P6Regex/Actions.nqp
index b5f3d3d0f4..cee2db9f1f 100644
--- a/src/QRegex/P6Regex/Actions.nqp
+++ b/src/QRegex/P6Regex/Actions.nqp
@@ -772,6 +772,13 @@ class QRegex::P6Regex::Actions is HLL::Actions {
                         QAST::Regex.new( :rxtype<cclass>, :name<.> ) ) !!
                     QAST::Regex.new( :rxtype<alt>, |@alts );
         }
+
+        if $qast.rxtype ne 'cclass' && +@($qast) == 0 {
+            $/.panic('Cannot have an empty character class. If you intended to include '
+                ~ 'whitespace within this character class, escape it with \\'
+            );
+        }
+
         make $qast;
     }