Lex Samples

This repository contains sample problems solved using lex/flex utility.

Prerequisites

Lex is a lexical analyser tool mostly used with Yacc parse generator. It lexically analyses (i.e. matches) the patterns (regular expressions) given as input string or as a file.

Following are the Lex predefined functions and variables :-

1. yyin: the input stream pointer (i.e it points to an input file which is to be scanned or tokenised), however the default input of default main() is stdin.
2. yylex(): implies the main entry point for lex, reads the input stream generates tokens, returns zero at the end of input stream. It is called to invoke the lexer (or scanner) and each time yylex() is called, the scanner continues processing the input from where it last left off.
3. yytext: a buffer that holds the input characters that actually match the pattern (i.e lexeme) or say a pointer to the matched string.
4. yyleng: the length of the lexeme.
5. yylval: contains the token value.
6. yyval: a local variable.
7. yyout: the output stream pointer (i.e it points to a file where it has to keep the output), however the default output of default main() is stdout.
8. yywrap(): it is called by lex when input is exhausted (or at EOF). default yywrap always return 1.
9. yymore(): returns the next token.
10. yyless(k): returns the first k characters in yytext.
11. yyparse(): it parses (i.e builds the parse tree) of lexeme.

Problem Statements

1. print "GOODBYE!" in place of "hello world" in a given string
2. count number of spaces in a given string
3. count number of words in a given string
4. count the number of vowels and consonents in a given string
   • approach 1
   • approach 2
5. count words that are less than 10 and greater than 5 in a given string
   • approach 1
   • approach 2
6. count the frequency of the given word in a file
7. find the length of the longest word in a given string
8. find the length of a given string
9. accept string starting with vowel
10. accept a valid integer and float value
11. find the number of valid 'printf' statements alongwith variables used in it

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