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(flex.info)YACC interface

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Interfacing with `yacc'
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   One of the main uses of `flex' is as a companion to the `yacc'
parser-generator.  `yacc' parsers expect to call a routine named
`yylex()' to find the next input token.  The routine is supposed to
return the type of the next token as well as putting any associated
value in the global `yylval'.  To use `flex' with `yacc', one specifies
the `-d' option to `yacc' to instruct it to generate the file `y.tab.h'
containing definitions of all the `%tokens' appearing in the `yacc'
input.  This file is then included in the `flex' scanner.  For example,
if one of the tokens is "TOK_NUMBER", part of the scanner might look
like:

     %{
     #include "y.tab.h"
     %}
     
     %%
     
     [0-9]+        yylval = atoi( yytext ); return TOK_NUMBER;