Definitions

The lex definitions section can contain any of several classes of items. The most critical are external definitions, preprocessor statements like #include, and abbreviations. For legal lex source this section is optional, but in most cases some of these items are necessary. Preprocessor statements and C source code appear between a line of the form %{ and one of the form %}.

All lines between these delimiters -- including those that begin with white space -- are copied to lex.yy.c immediately before the definition of yylex(). (Lines in the definition section that are not enclosed by the delimiters are copied to the same place provided they begin with white space.)

The definitions section is where you usually place C definitions of objects accessed by actions in the rules section or by routines with external linkage.

For example, when using lex with yacc, which generates parsers that call a lexical analyzer, include the file y.tab.h, which can contain #defines for token names:

%{ 
#include "y.tab.h"
extern int tokval; 
int lineno;
%} 

After the %} that ends your #includes and declarations, place your abbreviations for regular expressions in the rules section. The abbreviation appears on the left of the line and, separated by one or more spaces, its definition or translation appears on the right.

When you later use abbreviations in your rules, be sure to enclose them within braces. Abbreviations avoid repetition in writing your specifications and make them easier to read.

As an example, reconsider the lex source reviewed in the section " Advanced lex Features ". Using definitions simplifies later reference to digits, letters, and blanks.

This is especially true when the specifications appear several times:

D		             		[0-9]
L		             		[a-zA-Z]
B		             		[ \t]+
%%
-{D}+			          printf("negative integer");
\+?{D}+		    	   	printf("positive integer");
-0.{D}+ 	         printf("negative fraction"); 
G{L}*		    	     	printf("may have a G word
                          here"); 
rail{B}road			   	printf("railroad is one word");
crook		         		printf("criminal"); 
 ...
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