Examples taken from :-
http://ds9a.nl/lex-yacc/cvs/lex-yacc-howto.html

Just type "make" to build all the binaries.
"make clean" to  clean all the generated files.

Summary: 
1) lex parses a stream of strings and generates
tokens.i
2) Yacc parses tokens and takes actions if a rule 
is satisfied.
3) Whenever Lex matches a target, it puts the 
text of the match in the character string 'yytext'