Index: configure.in
===================================================================
--- configure.in	(revision 170018)
+++ configure.in	(working copy)
@@ -277,6 +277,44 @@
 MP_VERSION=`awk '/MPV_STRING/ {print $3}' src/include/mpversion.h`
 MP_VERSION=`echo $MP_VERSION | sed s/\\"//g`                                      
 
+# check for correct flex version
+# requires flex 2.5.31 for reentrant support
+
+AC_SUBST(LEX)
+
+AC_MSG_CHECKING(for --with-flex)
+AC_ARG_WITH(flex, [--with-flex=PATH	Path to specific flex binary],
+[
+  LEX="$withval"
+  AC_MSG_RESULT($LEX)
+],
+AC_MSG_RESULT(no))
+
+# check for flex executable
+if test -z "$LEX"; then
+  AC_PATH_PROG(LEX, flex)
+  if test -z "$LEX"; then
+    AC_MSG_ERROR(flex binary not found in path)
+  fi
+fi
+
+if test "$LEX" && test -x "$LEX"; then
+  AC_MSG_RESULT([found $LEX, we'll use this. Use --with-flex to specify another.])
+else
+  AC_MSG_ERROR([flex $LEX not found,  Use --with-flex to specify another.])
+fi
+
+AC_MSG_CHECKING(flex version)
+FlexVERSION=`$LEX --version | sed 's/version//g' | awk '/flex/ {print $2}'`
+Flex_MAJOR_VER=`echo $FlexVERSION| awk -F .  '{print $1}'`
+Flex_MINOR_VER=`echo $FlexVERSION| awk -F .  '{print $2}'`
+Flex_SUB_VER=`echo $FlexVERSION| awk -F .  '{print $3}'`
+if test "$Flex_MAJOR_VER" -eq "2" && test "$Flex_MINOR_VER" -eq "5" && test "$Flex_SUB_VER" -ge "31"; then
+  AC_MSG_RESULT([$FlexVERSION. Good])
+else
+  AC_MSG_WARN([Flex version 2.5.31 or greater is required.  The one you have seems to be $FlexVERSION. Use --with-flex to specify another.])
+fi
+
 AC_OUTPUT(Makefile src/Makefile Doc/Makefile test/testconf.py dist/setup.py dist/Makefile)
 
 
Index: src/Makefile.in
===================================================================
--- src/Makefile.in	(revision 170018)
+++ src/Makefile.in	(working copy)
@@ -21,7 +21,7 @@
 MKDEP=@MKDEP@
 
 # requires flex 2.5.31 for reentrant support
-LEX=/usr/local/bin/flex
+LEX=@LEX@
 INCLUDES=@INCLUDES@
 LIBS=@LIBS@
 LDFLAGS=@LDFLAGS@
Index: README
===================================================================
--- README	(revision 170018)
+++ README	(working copy)
@@ -6,6 +6,7 @@
 2. New in 3.0
 3. Migrating from Mod_Python 2.7.8
 4. OS Hints
+5. Flex version
 
 
 
@@ -151,3 +152,21 @@
 I don't know what the *right* solution for this is, but here is a
 trick that works: define DYLD_FORCE_FLAT_NAMESPACE environment
 variable prior to launching httpd.
+
+
+5. Flex version
+
+The parser used by psp is written in C which is generated using flex.
+Flex version 2.5.31, which is reentrant, is required to generate
+the src/psp_parser.c file from src/psp_parser.l. 
+
+Configure will detect the path and version of flex on your system 
+and generate a warning if the wrong version is found. Since most 
+current platforms ship with version 2.5.4, which is not reentrant,
+a pre-generated psp_parser.c has been provided, so you can
+safely ignore this warning when you run the configure script.
+
+However, if you make any changes to psp_parser.l, or psp_parser.c
+is missing, you will need to install the correct version of flex.
+Once installed, use configure --with-flex to specify the alternate
+location of flex, eg. ./configure --with-flex=/usr/local/bin/flex.
Index: Doc/modpython2.tex
===================================================================
--- Doc/modpython2.tex	(revision 170018)
+++ Doc/modpython2.tex	(working copy)
@@ -88,7 +88,7 @@
   your Python binary. By default, it will use the \program{python}
   program found in your \envvar{PATH}.
 
-  \indexii{./configure}{\longprogramopt{with-python}} If the fist Python
+  \indexii{./configure}{\longprogramopt{with-python}} If the first Python
   binary in the path is not suitable or not the one desired for
   mod_python, you can specify an alternative location with the
   \longprogramopt{with-python} options, e.g:
@@ -98,6 +98,31 @@
   \end{verbatim}                      
   %$ keep emacs happy
 
+\item
+  \index{flex}
+  Attempts to locate \program{flex} and determine its version. 
+  If \program{flex} cannot be found in your \envvar{PATH} \program{configure}
+  will fail.  If the wrong version is found \program{configure} will generate a warning.
+  You can generally ignore this warning unless you need to re-create
+  \filenq{src/psp_parser.c}.
+
+  The parser used by psp (See \ref{pyapi-psp}) is written in C generated using 
+  \program{flex}. This requires a reentrant version of \program{flex} which
+  at this time is 2.5.31. Most platforms however ship with version 2.5.4
+  which is not suitable, so a pre-generated copy of psp_parser.c is included
+  with the source. If you do need to compile \filenq{src/psp_parser.c} you 
+  must get the correct \program{flex} version.
+
+  \indexii{./configure}{\longprogramopt{with-flex}} If the first flex
+  binary in the path is not suitable or not the one desired you can
+  specify an alternative location with the \longprogramopt{with-flex}
+  options, e.g:
+
+  \begin{verbatim}
+    $ ./configure --with-flex=/usr/local/bin/flex
+  \end{verbatim}                      
+  %$ keep emacs happy
+
 \end{itemize}
 
 \subsection{Running make\label{inst-make}}
@@ -182,7 +207,7 @@
 \section{Testing\label{inst-testing}}
 
 \strong{Warning :} those instruction are meant to be followed if you are
-using mod_python 3.x or later.\If you are using mod_python 2.7.x (namely,
+using mod_python 3.x or later. If you are using mod_python 2.7.x (namely,
 if you are using Apache 1.3.x), please refer to the proper documentation.
 
 \begin{enumerate}