diff --git a/chapter01/changelog.xml b/chapter01/changelog.xml
index 26c2bfd22..d635048c1 100644
--- a/chapter01/changelog.xml
+++ b/chapter01/changelog.xml
@@ -45,6 +45,12 @@
     <listitem>
       <para>2020-04-19</para>
       <itemizedlist>
+        <listitem>
+          <para>[pierre] - Reinstate flex in chapter 5, so that ar and ranlib
+          (binutils) link against libfl. This allows to run tests for bison,
+          too. Fixes
+          <ulink url="&lfs-ticket-root;4631">#4631</ulink>.</para>
+        </listitem>
         <listitem>
           <para>[pierre] - Prevent readline.pc from referencing termcap as
           a private library by adding --with-curses to configure switches.
diff --git a/chapter05/chapter05.xml b/chapter05/chapter05.xml
index 3d82af568..2d02f9a2e 100644
--- a/chapter05/chapter05.xml
+++ b/chapter05/chapter05.xml
@@ -28,6 +28,7 @@
   <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="ncurses.xml"/>
   <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="bash.xml"/>
   <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="bison.xml"/>
+  <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="flex.xml"/>
   <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="bzip2.xml"/>
   <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="coreutils.xml"/>
   <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="diffutils.xml"/>
diff --git a/chapter05/flex.xml b/chapter05/flex.xml
new file mode 100644
index 000000000..5e1afa194
--- /dev/null
+++ b/chapter05/flex.xml
@@ -0,0 +1,72 @@
+<?xml version="1.0" encoding="ISO-8859-1"?>
+<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
+  <!ENTITY % general-entities SYSTEM "../general.ent">
+  %general-entities;
+]>
+
+<sect1 id="ch-tools-flex" role="wrap">
+  <?dbhtml filename="flex.html"?>
+
+  <sect1info condition="script">
+    <productname>flex</productname>
+    <productnumber>&flex-version;</productnumber>
+    <address>&flex-url;</address>
+  </sect1info>
+
+  <title>Flex-&flex-version;</title>
+
+  <indexterm zone="ch-tools-flex">
+    <primary sortas="a-Flex">Flex</primary>
+    <secondary>tools</secondary>
+  </indexterm>
+
+  <sect2 role="package">
+    <title/>
+
+    <xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
+    href="../chapter06/flex.xml"
+    xpointer="xpointer(/sect1/sect2[1]/para[1])"/>
+
+    <segmentedlist>
+      <segtitle>&buildtime;</segtitle>
+      <segtitle>&diskspace;</segtitle>
+
+      <seglistitem>
+        <seg>&flex-ch5-sbu;</seg>
+        <seg>&flex-ch5-du;</seg>
+      </seglistitem>
+    </segmentedlist>
+
+  </sect2>
+
+  <sect2 role="installation">
+    <title>Installation of Flex</title>
+
+    <para>Prepare Flex for compilation:</para>
+
+<screen><userinput remap="configure">./configure --prefix=/tools</userinput></screen>
+
+    <para>Compile the package:</para>
+
+<screen><userinput remap="make">make</userinput></screen>
+
+    <para>To test the results, issue:</para>
+
+<screen><userinput remap="test">make check</userinput></screen>
+
+    <para>Install the package:</para>
+
+<screen><userinput remap="install">make install</userinput></screen>
+
+  </sect2>
+
+  <sect2 role="content">
+    <title/>
+
+    <para>Details on this package are located in
+    <xref linkend="contents-flex" role="."/></para>
+
+  </sect2>
+
+</sect1>
diff --git a/chapter06/bison.xml b/chapter06/bison.xml
index a5ca3cfc0..e36e323eb 100644
--- a/chapter06/bison.xml
+++ b/chapter06/bison.xml
@@ -58,16 +58,19 @@
 
 <screen><userinput remap="make">make</userinput></screen>
 
-<!--
-    <para>To test the results (about 2.2 SBU), issue:</para>
+    <para>To test the results (about 5.5 SBU), issue:</para>
 
 <screen><userinput remap="test">make check</userinput></screen>
--->
 
+    <para>13 tests fail in the "Diagnostics" section, probably because of
+    missing locales.</para>
+
+<!--
     <para>There is a circular dependency between bison and flex with regard to
     the checks.  If desired, after installing flex in the next section, the
     bison package can be rebuilt and the bison checks can be run with 
     <command>make check</command>.</para>
+-->
 
     <para>Install the package:</para>
 
diff --git a/packages.ent b/packages.ent
index f13b0f20b..480ed66cf 100644
--- a/packages.ent
+++ b/packages.ent
@@ -77,8 +77,8 @@
 <!ENTITY bison-home "&gnu-software;bison/">
 <!ENTITY bison-ch5-du "43 MB">
 <!ENTITY bison-ch5-sbu "0.3 SBU">
-<!ENTITY bison-ch6-du "43 MB">
-<!ENTITY bison-ch6-sbu "0.3 SBU">
+<!ENTITY bison-ch6-du "52 MB">
+<!ENTITY bison-ch6-sbu "5.8 SBU">
 
 <!ENTITY bzip2-version "1.0.8">
 <!ENTITY bzip2-size "792 KB">
@@ -209,6 +209,8 @@
 <!ENTITY flex-url "&github;/westes/flex/releases/download/v&flex-version;/flex-&flex-version;.tar.gz">
 <!ENTITY flex-md5 "2882e3179748cc9f9c23ec593d6adc8d">
 <!ENTITY flex-home "&github;/westes/flex">
+<!ENTITY flex-ch5-du "15 MB">
+<!ENTITY flex-ch5-sbu "0.1 SBU">
 <!ENTITY flex-ch6-du "36 MB">
 <!ENTITY flex-ch6-sbu "0.4 SBU">