Suppress "yyunput defined but not used"

To suppress the warning message "yyunput defined but not used" when compiling the flex output with gcc -Wall, use a line

  %option nounput

Make flex generate line numbers

  %option yylineno

Make flex create a header file declaring yylex

  flex --header-file=my.lex.h my.l

Change the yy prefix to something else

  %option prefix="myprefix_"

Stop the "undefined reference to `yywrap'" error

  %option noyywrap

Stop the "warning: 'yy_top_state' defined but not used" error

  %option noyy_top_state

read string as input instead of file pointer

 # in scanner
 YY_BUFFER_STATE str_buffer;

 void scan_begin(const char * cmd)
  str_buffer = cmdline_scan_string(cmd);

 void scan_end ()

  # in main:
  // cmdline parsing
  ostringstream oss;
  for(int i=1;i<argc;i++) oss << argv[i] << " ";

Vairables in flex

functions in flex

If return value is 1 then flex will be abadoned. If return value is 0 then yywrap can append another input stream.

predefined macros

start conditions

%s asdf %S asdf %start asdf %swhatever asdf

defines 'asdf' as start rule which you can use with command 'begin'
 begin 0
changes into normal mode

 %s replace
 ^#                  { ECHO; BEGIN replace; }
 \n                  { ECHO; BEGIN 0;  }
 <replace>PATTERN    printf("asdf");

This example takes the input and if a line starts with # the normal mode switchs to rule 'replace' and if any string PATTERN would be found it will be replaced by 'asdf'. After the linefeed it would switch back to normal mode.

meta characters

 \ ^  $ . [ ] | ( ) * + ? { } " / % < >

classes of characters


all characters except one between A and Z


meets string as he, she or it
 [tT]he\ (good|bad)\ story.
meets "The good story" and "the bad story".


all three expresions meet "abc++".

one,more or nothing

 (r)* no or more occurrences of regular expression r. The empty string also meets this.
 (r)+ one or more occurrences of regular expression r.
 (r)? no or one occurrences of r.

The brackets define a part of the string as combined.


meets double vocals.
meets occurences between n1 and n2.

meet line patterns with \n (linefeed)


Both regular expressions meet a line of the input but the difference lies in the last character \n. In the first case the linefeed does not go into yytext. In the second case it goes into yytext.

flex with bison


 #define IF 4711
 #define ELSE 4712

which can be included in the definition part of the lex file:

 # include "y.tag.h"

Steps are:

  1. bison -d example.y
creates y.tab.c and y.tab.h
  1. flex example.lex
generates lex.yy.c
  1. gcc lex.yy.c y.tab.c -lfl

applications of flex