--- a/libinterp/interpfcn/input.cc	Wed Feb 27 18:21:50 2013 -0500
+++ b/libinterp/interpfcn/input.cc	Wed Feb 27 18:49:16 2013 -0500
@@ -667,29 +667,18 @@
 
       frame.protect_var (get_input_from_eval_string);
       get_input_from_eval_string = false;
-
-      YY_BUFFER_STATE old_buf = current_buffer ();
-      YY_BUFFER_STATE new_buf = create_buffer (get_input_from_stdin ());
-
-      // FIXME: are these safe?
-      frame.add_fcn (switch_to_buffer, old_buf);
-      frame.add_fcn (delete_buffer, new_buf);
-
-      switch_to_buffer (new_buf);
     }
 
-  frame.protect_var (curr_lexer);
-  curr_lexer = new lexical_feedback ();
-  frame.add_fcn (lexical_feedback::cleanup, curr_lexer);
-
-  frame.protect_var (curr_parser);
-  curr_parser = new octave_parser ();
-  frame.add_fcn (octave_parser::cleanup, curr_parser);
-
   while (Vdebugging)
     {
       unwind_protect middle_frame;
 
+      // octave_parser constructor sets this for us.
+      middle_frame.protect_var (CURR_LEXER);
+
+      octave_parser *curr_parser = new octave_parser ();
+      middle_frame.add_fcn (octave_parser::cleanup, curr_parser);
+
       reset_error_handler ();
 
       curr_parser->reset ();
@@ -1199,33 +1188,28 @@
 static int
 input_event_hook (void)
 {
-  if (! curr_lexer->defining_func)
-    {
-      hook_fcn_map_type::iterator p = hook_fcn_map.begin ();
+  hook_fcn_map_type::iterator p = hook_fcn_map.begin ();
 
-      while (p != hook_fcn_map.end ())
-        {
-          std::string hook_fcn = p->first;
-          octave_value user_data = p->second;
+  while (p != hook_fcn_map.end ())
+    {
+      std::string hook_fcn = p->first;
+      octave_value user_data = p->second;
 
-          hook_fcn_map_type::iterator q = p++;
+      hook_fcn_map_type::iterator q = p++;
 
-          if (is_valid_function (hook_fcn))
-            {
-              if (user_data.is_defined ())
-                feval (hook_fcn, user_data, 0);
-              else
-                feval (hook_fcn, octave_value_list (), 0);
-            }
+      if (is_valid_function (hook_fcn))
+        {
+          if (user_data.is_defined ())
+            feval (hook_fcn, user_data, 0);
           else
-            hook_fcn_map.erase (q);
+            feval (hook_fcn, octave_value_list (), 0);
         }
-
-      if (hook_fcn_map.empty ())
-        command_editor::remove_event_hook (input_event_hook);
+      else
+        hook_fcn_map.erase (q);
     }
 
-  return 0;
+  if (hook_fcn_map.empty ())
+    command_editor::remove_event_hook (input_event_hook);
 }
 
 DEFUN (add_input_event_hook, args, ,

--- a/libinterp/interpfcn/toplev.cc	Wed Feb 27 18:21:50 2013 -0500
+++ b/libinterp/interpfcn/toplev.cc	Wed Feb 27 18:49:16 2013 -0500
@@ -557,16 +557,6 @@
 
   octave_initialized = true;
 
-  unwind_protect frame;
-
-  frame.protect_var (curr_lexer);
-  curr_lexer = new lexical_feedback ();
-  frame.add_fcn (lexical_feedback::cleanup, curr_lexer);
-
-  frame.protect_var (curr_parser);
-  curr_parser = new octave_parser ();
-  frame.add_fcn (octave_parser::cleanup, curr_parser);
-
   // The big loop.
 
   int retval = 0;
@@ -574,7 +564,13 @@
     {
       try
         {
-          unwind_protect inner_frame;
+          unwind_protect frame;
+
+          // octave_parser constructor sets this for us.
+          frame.protect_var (CURR_LEXER);
+
+          octave_parser *curr_parser = new octave_parser ();
+          frame.add_fcn (octave_parser::cleanup, curr_parser);
 
           reset_error_handler ();
 

--- a/libinterp/octave.cc	Wed Feb 27 18:21:50 2013 -0500
+++ b/libinterp/octave.cc	Wed Feb 27 18:49:16 2013 -0500
@@ -997,9 +997,6 @@
   // Now argv should have the full set of args.
   intern_argv (octave_cmdline_argc, octave_cmdline_argv);
 
-  if (! octave_embedded)
-    switch_to_buffer (create_buffer (get_input_from_stdin ()));
-
   // Force input to be echoed if not really interactive, but the user
   // has forced interactive behavior.
 

--- a/libinterp/parse-tree/lex.h	Wed Feb 27 18:21:50 2013 -0500
+++ b/libinterp/parse-tree/lex.h	Wed Feb 27 18:49:16 2013 -0500
@@ -27,25 +27,6 @@
 #include <set>
 #include <stack>
 
-// FIXME -- these input buffer things should be members of a
-// parser input stream class.
-
-typedef struct yy_buffer_state *YY_BUFFER_STATE;
-
-// Associate a buffer with a new file to read.
-extern OCTINTERP_API YY_BUFFER_STATE create_buffer (FILE *f);
-
-// Report the current buffer.
-extern OCTINTERP_API YY_BUFFER_STATE current_buffer (void);
-
-// Connect to new buffer buffer.
-extern OCTINTERP_API void switch_to_buffer (YY_BUFFER_STATE buf);
-
-// Delete a buffer.
-extern OCTINTERP_API void delete_buffer (YY_BUFFER_STATE buf);
-
-extern OCTINTERP_API void clear_all_buffers (void);
-
 extern OCTINTERP_API void cleanup_parser (void);
 
 // Is the given string a keyword?
@@ -173,8 +154,8 @@
   };
 
   lexical_feedback (void)
-    : convert_spaces_to_comma (true), do_comma_insert (false),
-      at_beginning_of_statement (true),
+    : scanner (0), convert_spaces_to_comma (true),
+      do_comma_insert (false), at_beginning_of_statement (true),
       looking_at_anon_fcn_args (false), looking_at_return_list (false),
       looking_at_parameter_list (false), looking_at_decl_list (false),
       looking_at_initializer_expression (false),
@@ -195,12 +176,7 @@
 
   ~lexical_feedback (void);
 
-  void init (void)
-  {
-    // The closest paren, brace, or bracket nesting is not an object
-    // index.
-    looking_at_object_index.push_front (false);
-  }
+  void init (void);
 
   void reset (void);
 
@@ -297,6 +273,9 @@
 
   void lexer_debug (const char *pattern, const char *text);
 
+  // Internal state of the flex-generated lexer.
+  void *scanner;
+
   // TRUE means that we should convert spaces to a comma inside a
   // matrix definition.
   bool convert_spaces_to_comma;
@@ -407,7 +386,4 @@
   lexical_feedback& operator = (const lexical_feedback&);
 };
 
-// The current state of the lexer.
-extern lexical_feedback *curr_lexer;
-
 #endif

--- a/libinterp/parse-tree/lex.ll	Wed Feb 27 18:21:50 2013 -0500
+++ b/libinterp/parse-tree/lex.ll	Wed Feb 27 18:49:16 2013 -0500
@@ -20,8 +20,19 @@
 
 */
 
+// We are using the pure parser interface and the reentrant lexer
+// interface but the Octave parser and lexer are NOT properly
+// reentrant because both still use many global variables.  It should be
+// safe to create a parser object and call it while anotehr parser
+// object is active (to parse a callback function while the main
+// interactive parser is waiting for input, for example) if you take
+// care to properly save and restore (typically with an unwind_protect
+// object) relevant global values before and after the nested call.
+
 %option prefix = "octave_"
 %option noyywrap
+%option reentrant
+%option bison-bridge
 
 %top {
 #ifdef HAVE_CONFIG_H
@@ -96,7 +107,8 @@
 #error lex.l requires flex version 2.5.4 or later
 #endif
 
-#define yylval octave_lval
+#define YY_EXTRA_TYPE lexical_feedback *
+#define curr_lexer yyextra
 
 // Arrange to get input via readline.
 
@@ -107,14 +119,12 @@
   result = curr_lexer->octave_read (buf, max_size)
 
 // Try to avoid crashing out completely on fatal scanner errors.
-// The call to yy_fatal_error should never happen, but it avoids a
-// 'static function defined but not used' warning from gcc.
 
 #ifdef YY_FATAL_ERROR
 #undef YY_FATAL_ERROR
 #endif
 #define YY_FATAL_ERROR(msg) \
-  curr_lexer->fatal_error (msg)
+  (yyget_extra (yyscanner))->fatal_error (msg)
 
 #define DISPLAY_TOK_AND_RETURN(tok) \
   do \
@@ -206,9 +216,6 @@
     } \
   while (0)
 
-// The state of the lexer.
-lexical_feedback *curr_lexer = 0;
-
 static bool Vdisplay_tokens = false;
 
 static unsigned int Vtoken_count = 0;
@@ -1117,70 +1124,9 @@
       }
 }
 
-// Tell us all what the current buffer is.
-
-YY_BUFFER_STATE
-current_buffer (void)
-{
-  return YY_CURRENT_BUFFER;
-}
-
-// Create a new buffer.
-
-YY_BUFFER_STATE
-create_buffer (FILE *f)
-{
-  return yy_create_buffer (f, YY_BUF_SIZE);
-}
-
-// Start reading a new buffer.
-
-void
-switch_to_buffer (YY_BUFFER_STATE buf)
-{
-  yy_switch_to_buffer (buf);
-}
-
-// Delete a buffer.
-
-void
-delete_buffer (YY_BUFFER_STATE buf)
-{
-  yy_delete_buffer (buf);
-
-  // Prevent invalid yyin from being used by yyrestart.
-  if (! current_buffer ())
-    yyin = 0; 
-}
-
-// Delete all buffers from the stack.
-void
-clear_all_buffers (void)
-{                 
-  while (current_buffer ())
-    octave_pop_buffer_state ();
-}
-
 void
 cleanup_parser (void)
 {
-  clear_all_buffers ();
-}
-
-// Restore a buffer (for unwind-prot).
-
-void
-restore_input_buffer (void *buf)
-{
-  switch_to_buffer (static_cast<YY_BUFFER_STATE> (buf));
-}
-
-// Delete a buffer (for unwind-prot).
-
-void
-delete_input_buffer (void *buf)
-{
-  delete_buffer (static_cast<YY_BUFFER_STATE> (buf));
 }
 
 // Return 1 if the given character matches any character in the given
@@ -1366,11 +1312,38 @@
       delete token_stack.top ();
       token_stack.pop ();
     }
+
+  yylex_destroy (scanner);
 }
 
 void
+lexical_feedback::init (void)
+{
+  // The closest paren, brace, or bracket nesting is not an object
+  // index.
+  looking_at_object_index.push_front (false);
+
+  yylex_init (&scanner);
+
+  // Make lexical_feedback object available through yyextra in
+  // flex-generated lexer.
+  yyset_extra (this, scanner);
+}
+
+// Inside Flex-generated functions, yyg is the scanner cast to its real
+// type.  The BEGIN macro uses yyg and we want to use that in
+// lexical_feedback member functions.  If we could set the start state
+// by calling a function instead of using the BEGIN macro, we could
+// eliminate the OCTAVE_YYG macro.
+
+#define OCTAVE_YYG \
+  struct yyguts_t *yyg = static_cast<struct yyguts_t*> (scanner)
+
+void
 lexical_feedback::reset (void)
 {
+  OCTAVE_YYG;
+
   // Start off on the right foot.
   BEGIN (INITIAL);
 
@@ -1389,7 +1362,7 @@
             || reading_script_file
             || get_input_from_eval_string
             || input_from_startup_file))
-    yyrestart (stdin);
+    yyrestart (stdin, scanner);
 
   // Clear the buffer for help text.
   while (! help_buf.empty ())
@@ -1399,12 +1372,16 @@
 void
 lexical_feedback::prep_for_script_file (void)
 {
+  OCTAVE_YYG;
+
   BEGIN (SCRIPT_FILE_BEGIN);
 }
 
 void
 lexical_feedback::prep_for_function_file (void)
 {
+  OCTAVE_YYG;
+
   BEGIN (FUNCTION_FILE_BEGIN);
 }
 
@@ -1466,7 +1443,7 @@
       status = YY_NULL;
 
       if (! eof)
-        YY_FATAL_ERROR ("octave_read () in flex scanner failed");
+        fatal_error ("octave_read () in flex scanner failed");
     }
 
   return status;
@@ -1475,13 +1452,13 @@
 char *
 lexical_feedback::flex_yytext (void)
 {
-  return yytext;
+  return yyget_text (scanner);
 }
 
 int
 lexical_feedback::flex_yyleng (void)
 {
-  return yyleng;
+  return yyget_leng (scanner);
 }
 
 // GAG.
@@ -1508,7 +1485,7 @@
 int
 lexical_feedback::text_yyinput (void)
 {
-  int c = yyinput ();
+  int c = yyinput (scanner);
 
   if (lexer_debug_flag)
     {
@@ -1521,7 +1498,7 @@
 
   if (c == '\r')
     {
-      c = yyinput ();
+      c = yyinput (scanner);
 
       if (lexer_debug_flag)
         {
@@ -1556,13 +1533,14 @@
   if (c == '\n')
     input_line_number--;
 
-  yyunput (c, buf);
+  yyunput (c, buf, scanner);
 }
 
 void
 lexical_feedback::xunput (char c)
 {
   char *yytxt = flex_yytext ();
+
   xunput (c, yytxt);
 }
 
@@ -2077,6 +2055,8 @@
 int
 lexical_feedback::process_comment (bool start_in_block, bool& eof)
 {
+  OCTAVE_YYG;
+
   eof = false;
 
   std::string help_txt;
@@ -2839,6 +2819,8 @@
 int
 lexical_feedback::handle_close_bracket (bool spc_gobbled, int bracket_type)
 {
+  OCTAVE_YYG;
+
   int retval = bracket_type;
 
   if (! nesting_level.none ())
@@ -3283,6 +3265,8 @@
 int
 lexical_feedback::handle_identifier (void)
 {
+  OCTAVE_YYG;
+
   bool at_bos = at_beginning_of_statement;
 
   char *yytxt = flex_yytext ();
@@ -3518,14 +3502,16 @@
 void
 lexical_feedback::push_token (token *tok)
 {
-  yylval.tok_val = tok;
+  YYSTYPE *lval = yyget_lval (scanner);
+  lval->tok_val = tok;
   token_stack.push (tok);
 }
 
 token *
 lexical_feedback::current_token (void)
 {
-  return yylval.tok_val;
+  YYSTYPE *lval = yyget_lval (scanner);
+  return lval->tok_val;
 }
 
 void
@@ -3706,12 +3692,14 @@
 
   OCTAVE_QUIT;
 
-  yy_fatal_error (msg);
+  yy_fatal_error (msg, scanner);
 }
 
 void
 lexical_feedback::lexer_debug (const char *pattern, const char *text)
 {
+  OCTAVE_YYG;
+
   std::cerr << std::endl;
 
   display_state (YY_START);

--- a/libinterp/parse-tree/oct-parse.yy	Wed Feb 27 18:21:50 2013 -0500
+++ b/libinterp/parse-tree/oct-parse.yy	Wed Feb 27 18:49:16 2013 -0500
@@ -77,6 +77,15 @@
 #include "utils.h"
 #include "variables.h"
 
+// oct-parse.h must be included after pt-all.h
+#include <oct-parse.h>
+
+extern int octave_lex (YYSTYPE *, void *);
+
+// Global access to currently active lexer.
+// FIXME -- to be removed after more parser+lexer refactoring.
+lexical_feedback *CURR_LEXER = 0;
+
 #if defined (GNULIB_NAMESPACE)
 // Calls to the following functions appear in the generated output from
 // Bison without the namespace tag.  Redefine them so we will use them
@@ -86,9 +95,6 @@
 #define malloc GNULIB_NAMESPACE::malloc
 #endif
 
-// The state of the parser.
-octave_parser *curr_parser = 0;
-
 // Buffer for help text snagged from function files.
 std::stack<std::string> help_buf;
 
@@ -155,7 +161,7 @@
 // Forward declarations for some functions defined at the bottom of
 // the file.
 
-static void yyerror (const char *s);
+static void yyerror (octave_parser *curr_parser, const char *s);
 
 // Finish building a statement.
 template <class T>
@@ -182,6 +188,9 @@
     } \
   while (0)
 
+#define curr_lexer curr_parser->curr_lexer
+#define scanner curr_lexer->scanner
+
 %}
 
 // Bison declarations.
@@ -191,6 +200,19 @@
 
 %name-prefix="octave_"
 
+// We are using the pure parser interface and the reentrant lexer
+// interface but the Octave parser and lexer are NOT properly
+// reentrant because both still use many global variables.  It should be
+// safe to create a parser object and call it while anotehr parser
+// object is active (to parse a callback function while the main
+// interactive parser is waiting for input, for example) if you take
+// care to properly save and restore (typically with an unwind_protect
+// object) relevant global values before and after the nested call.
+
+%define api.pure
+%parse-param { octave_parser *curr_parser }
+%lex-param { void *scanner }
+
 %union
 {
   // The type of the basic tokens returned by the lexer.
@@ -1493,8 +1515,10 @@
 
 // Generic error messages.
 
+#undef curr_lexer
+
 static void
-yyerror (const char *s)
+yyerror (octave_parser *curr_parser, const char *s)
 {
   curr_parser->bison_error (s);
 }
@@ -1502,7 +1526,7 @@
 int
 octave_parser::run (void)
 {
-  return octave_parse ();
+  return octave_parse (this);
 }
 
 // Error mesages for mismatched end tokens.
@@ -3281,7 +3305,7 @@
       if (eof)
         break;
 
-      txt = curr_lexer->grab_comment_block (stdio_reader, true, eof);
+      txt = CURR_LEXER->grab_comment_block (stdio_reader, true, eof);
 
       if (txt.empty ())
         break;
@@ -3377,17 +3401,19 @@
     {
       bool eof;
 
-      frame.protect_var (curr_lexer);
-      curr_lexer = new lexical_feedback ();
-      frame.add_fcn (lexical_feedback::cleanup, curr_lexer);
-
-      frame.protect_var (curr_parser);
-      curr_parser = new octave_parser ();
+      // octave_parser constructor sets this for us.
+      frame.protect_var (CURR_LEXER);
+
+      octave_parser *curr_parser = new octave_parser ();
       frame.add_fcn (octave_parser::cleanup, curr_parser);
 
       curr_parser->reset ();
 
-      std::string help_txt = gobble_leading_white_space (ffile, eof);
+      std::string help_txt
+        = gobble_leading_white_space
+            (ffile, eof,
+             curr_parser->curr_lexer->input_line_number,
+             curr_parser->curr_lexer->current_input_column);
 
       if (! help_txt.empty ())
         help_buf.push (help_txt);
@@ -3439,14 +3465,6 @@
               reading_script_file = true;
             }
 
-          YY_BUFFER_STATE old_buf = current_buffer ();
-          YY_BUFFER_STATE new_buf = create_buffer (ffile);
-
-          frame.add_fcn (switch_to_buffer, old_buf);
-          frame.add_fcn (delete_buffer, new_buf);
-
-          switch_to_buffer (new_buf);
-
           frame.protect_var (primary_fcn_ptr);
           primary_fcn_ptr = 0;
 
@@ -3460,11 +3478,11 @@
             help_buf.push (help_txt);
 
           if (reading_script_file)
-            curr_lexer->prep_for_script_file ();
+            curr_parser->curr_lexer->prep_for_script_file ();
           else
-            curr_lexer->prep_for_function_file ();
-
-          curr_lexer->parsing_class_method = ! dispatch_type.empty ();
+            curr_parser->curr_lexer->prep_for_function_file ();
+
+          curr_parser->curr_lexer->parsing_class_method = ! dispatch_type.empty ();
 
           frame.protect_var (global_command);
 
@@ -3486,9 +3504,11 @@
         }
       else
         {
+          int l = curr_parser->curr_lexer->input_line_number;
+          int c = curr_parser->curr_lexer->current_input_column;
+
           tree_statement *end_of_script
-            = curr_parser->make_end ("endscript", curr_lexer->input_line_number,
-                                     curr_lexer->current_input_column);
+            = curr_parser->make_end ("endscript", l, c);
 
           curr_parser->make_script (0, end_of_script);
 
@@ -4187,12 +4207,10 @@
 
   unwind_protect frame;
 
-  frame.protect_var (curr_lexer);
-  curr_lexer = new lexical_feedback ();
-  frame.add_fcn (lexical_feedback::cleanup, curr_lexer);
-
-  frame.protect_var (curr_parser);
-  curr_parser = new octave_parser ();
+  // octave_parser constructor sets this for us.
+  frame.protect_var (CURR_LEXER);
+
+  octave_parser *curr_parser = new octave_parser ();
   frame.add_fcn (octave_parser::cleanup, curr_parser);
 
   frame.protect_var (get_input_from_eval_string);
@@ -4220,14 +4238,6 @@
 
   current_eval_string = s;
 
-  YY_BUFFER_STATE old_buf = current_buffer ();
-  YY_BUFFER_STATE new_buf = create_buffer (0);
-
-  frame.add_fcn (switch_to_buffer, old_buf);
-  frame.add_fcn (delete_buffer, new_buf);
-
-  switch_to_buffer (new_buf);
-
   do
     {
       curr_parser->reset ();

--- a/libinterp/parse-tree/parse.h	Wed Feb 27 18:21:50 2013 -0500
+++ b/libinterp/parse-tree/parse.h	Wed Feb 27 18:49:16 2013 -0500
@@ -32,8 +32,6 @@
 #include "lex.h"
 #include "token.h"
 
-extern int octave_lex (void);
-
 class octave_comment_list;
 class octave_function;
 class octave_user_function;
@@ -132,14 +130,25 @@
 
 extern OCTINTERP_API void cleanup_statement_list (tree_statement_list **lst);
 
+// Global access to currently active lexer.
+// FIXME -- to be removed after more parser+lexer refactoring.
+extern lexical_feedback *CURR_LEXER;
+
 class
 octave_parser
 {
 public:
 
-  octave_parser (void) : end_of_input (false) { }
+  octave_parser (void)
+    : end_of_input (false), curr_lexer (new lexical_feedback ())
+  {
+    CURR_LEXER = curr_lexer;
+  }
 
-  ~octave_parser (void) { }
+  ~octave_parser (void)
+  {
+    delete curr_lexer;
+  }
 
   void reset (void)
   {
@@ -332,6 +341,9 @@
   // TRUE means that we have encountered EOF on the input stream.
   bool end_of_input;
 
+  // State of the lexer.
+  lexical_feedback *curr_lexer;
+
   // For unwind protect.
   static void cleanup (octave_parser *parser) { delete parser; }
 
@@ -344,7 +356,4 @@
   octave_parser& operator = (const octave_parser&);
 };
 
-// The current state of the parser.
-extern octave_parser *curr_parser;
-
 #endif