Write a simple lex specification to recognize the identifier.
Experiment Name
:- Write a simple lex specification to recognize
the identifier.
Theory
Scanner
:-
Conceptually, a compiler operates in
phases, each of which transforms the source.
Program from one representation to
another. The lexical analyzer is the first phase of a compiler. Its main task
is to read the input characters & produce as output a sequence of tokens
that the parser uses for syntax analysis. The lexical analyzer generator is
also known as lexer/scanner.
Interaction
with the parser :-
The lexical analyzer is an independent compilation phase that communicates with the parser over a well-defined and simple interface. The interaction, summarized in the following figure, is commonly implemented by making the lexical analyzer be a subroutine of the parser.
Lex
:-
Lex is a tool for gathering scanners.
LEX reads the given input files or standard input for a description of a
scanner to generate. The description is in the form of pairs of regular
expressions and C code, called RULES. So the LEX specification is given below:
Declarations
%%
Transition rules
%%
Auxiliary
procedures / Compiler :-
Here we use the LINUX Operating
Systems FLEX to write the LEX code. Where FLEX stands for-Fast Lexical Analyzer
generator. FLEX generates as output a C source file, lex.yy.c which defines a
routine yylex(), this file is compiled and linked with the – lfl library to
produce an exe file. When the executable is run, it analyzes its input for
occurrences of the regular expression. Whenever it finds one, it executes the
corresponding C code. So the command line can be as follows:
Flex test.l
Gcc lex.yy.c – lfl
./a.out
Identifier
:-
Languages use identification as names
of variables, arrays, functions and the like. A grammar for a language often
treats an identifier as a token. A parser based on such a grammar ants to see
the same token say id, each time an identifier appears in the input. For
example:
Count = count +
increment;
Would be converted by the lexical
analyzer into the token stream Id = id+id;
Keywords
:-
Many language use fixed character
strings such as begin, end, if, and so on, as punctuation marks or to identify
certain constructs. These character strings, called keywords, generally satisfy
the rules for forming identifiers, so a mechanism is needed for deciding when a
lexeme forms a keyword and when it forms an identifier. The problem is easier
to resolve if keywords are reserved, i.e. if they are cannot be used as
identifiers. Then a character string forms an identifier only if it is not a
keywords.
Implementation
Environment
Hardware
Information :-
Processor
Cyrix GX-Media
Speed
233 MHz
External Cache Memory 512 KB
Internal Cache Memory 8 KB
RAM 32 MB
Hard Disk Drive 8.6 GB
Operating System Linux
(Red Hat 5.2)
Program
Listing :-
%%
[a-zA-Z][a-zA-Z0-9_]* {printf(“Identifier\n”);}
.* {printf(“Not an Identifier\n”);}
%%
int yywrap(){}
int main()
{
printf(“Enter string to check
Identifier or not:\n”);
yylex();
}
Input and Output
Input
:-
Enter string to check Identifier or
not:
global
gtb458
bee see
573he
Output
:-
global
Identifier
gtb458
Identifier
bee see
Not an Identifier
573he
Not an Identifier
Discussion
Advantages
:-
In this program we can identify some
predefined verbs and can easily feed the output of this program to a parser
program. It helps us to understand how a scanner works and also how it interact
with the parser.
Limitations
:-
This program used only some of the
reserved keywords and works on C like statement.
Efficiency
:-
Under a little limitation the
efficiency of this program is 100%.
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