diff --git a/src/tools/msvc/pgflex.bat b/src/tools/msvc/pgflex.bat
index da7c33d7af..58870f0512 100755
--- a/src/tools/msvc/pgflex.bat
+++ b/src/tools/msvc/pgflex.bat
@@ -25,9 +25,19 @@ if "%1" == "contrib\seg\segscan.l" call :generate %1 contrib\seg\segscan.c
 echo Unknown flex input: %1
 exit 1
 
+REM For non-reentrant scanners we need to fix up the yywrap macro definition
+REM to keep the MS compiler happy.
+REM For reentrant scanners (like the core scanner) we do not
+REM need to (and must not) change the yywrap definition.
 :generate
 flex %3 -o%2 %1
-exit %errorlevel%
+if errorlevel 1 exit %errorlevel%
+perl -n -e "exit 1 if /^\%%option\s+reentrant/;" %1
+if errorlevel 1 exit 0
+perl -pi.bak -e "s/yywrap\(n\)/yywrap()/;" %2
+if errorlevel 1 exit %errorlevel%
+del %2.bak
+exit 0
 
 :noflex
 echo WARNING! flex install not found, attempting to build without