# HG changeset patch
# User Christian Urban <christian.urban@kcl.ac.uk>
# Date 1726770112 -3600
# Node ID 85bb0ef99fc7e4dadc48707ee012cf67ec16aae3
# Parent  5176cbb819c212c7d5027d5fe400d9641cb8c5ce
 texupdate

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diff -r 5176cbb819c2 -r 85bb0ef99fc7 hws/hw05.tex
--- a/hws/hw05.tex	Thu Sep 19 16:32:26 2024 +0100
+++ b/hws/hw05.tex	Thu Sep 19 19:21:52 2024 +0100
@@ -56,6 +56,22 @@
 %where the three dots stand for arbitrary characters, but not
 %comment delimiters.
 
+\item The \emph{not}-regular expression is definitely useful for recognising
+  comments for example, but can
+  sometimes be quite unintuitive when it comes to deciding which strings are
+  matched or not. Consider
+
+  \[
+  (\sim{}a)^*  \quad\text{and}\quad \sim{}(a^*)\;.  
+  \]  
+
+  What is the language of each regular expression?
+
+  \solution{
+    The first one is ``all strings, except $[a]$''; the second
+    ``all strings except strings of the form $a^*$''.
+  }
+  
 \item Define the following regular expressions 
 
 \begin{center}
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