What are three parts of Lex program?

                     LEX program consists of three sections 
1. "Definitions", 
2. "Regular Expressions and action for each regular expression" 
3. "Subroutines".  
             
                Definition section consists of C language code which involves header file inclusion, global variables declaration/definition etc. C language code can be mentioned in between the symbols %{ and %}

                 LEX requires regular expressions or patterns to identify token of lexemes. Examples of token are identifier, header file, constants etc. while Lexemes are the actual words used in your input program for e.g. printf, scanf, stdio.h etc. These regular expressions and action for them are mentioned in second section. When we call yylex() function, it starts the process of pattern matching. Lex keep the matched string into the address pointed by pointer yytext. Matched string's length is kept in yyleng while value of token is kept in variable yylval. In above program NUMBER is a token. atoi() is a standard C function used to convert string to integer value. 

               Third section consists of subroutines/functions. Lex call yywrap() function after input is over. It should return 1 when work is done or should return 0 when more processing is required. yylex() function actual starts the process of pattern matching. In above program, we have not called yylex() since yyparse() in yacc program automatically calls yylex(). Thats why there in no need to call yylex() separately. If you are writing standalone Lex program, then you have to call yylex() in main() function in Lex program.