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 ()
 {
  cmdline_flush_buffer(str_buffer);
  cmdline_delete_buffer(str_buffer); 
 }

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

Vairables in flex

yytext points on the actual textstring which fits a specific regular expression in you flex file.
yyleng number of characters in yytext
yyin points by default to stdin
yyout points by default to stdout

functions in flex

yymore multiple use of a pattern
yyless push back string which was read
input read next character from input stream
output(c) give c to stdout
unput(c) push back c into input
yywrap() actions after reaching the end of the input stream.

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

predefined macros

ECHO gives out the content of yytext.

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

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

^ begin of line
$ end of line
. arbitrary character except \n
\n linefeed. See below for further explanations.

classes of characters

complement

 [^A-Z]

all characters except one between A and Z

alternation

 he|she|it

meets string as he, she or it

 [tT]he\ (good|bad)\ story.

meets "The good story" and "the bad story".

concatenation

 abc"++"
 "abc++"
 abc\+\+

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.

repitions

 [AEIOUaeiou]{2,2}

meets double vocals.

 (reg){n1,n2}

meets occurences between n1 and n2.

meet line patterns with \n (linefeed)

 ^.*$
 ^.*\n

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

paramters:

-d: Write an extra output file containing macro definitions for the token type names defined in the grammar and the semantic value type YYSTYPE, as well as a few extern variable declarations. This creates a file y.tab.h with:

 #define IF 4711
 #define ELSE 4712
 .
 .

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

 %{
 # include "y.tag.h"
 %}

Steps are:

bison -d example.y

creates y.tab.c and y.tab.h

flex example.lex

generates lex.yy.c

gcc lex.yy.c y.tab.c -lfl

Flex