More on LEX

LEX

In addition to scanner development, Lex/Flex is also used for

some other applications in system administration where text

processing is needed.

Patterns

r? zero or one occurrence of r

r{2,5} two to five occurrences of r

r{2,} two or more occurrences of r

r{4} exactly 4 occurrence of r

\0 a NUL character (ASCII code 0)

\123 the character with octal value 123

\x2a the character with hexadecimal value 2a

<s>r an r, but only in start condition s

<s1,s2,s3>r same, but in any of start conditions s1,

s2, or s3

<*>r an r in any start condition, even an

exclusive one.

<<EOF>> an end-of-file

<s1,s2><<EOF>> an end-of-file when in start condition s1

or s2

Patterns

Special Directives

ECHO copies yytext to the scanner's output.

BEGIN followed by the name of a start condition places the

scanner in the corresponding start condition.

REJECT directs the scanner to proceed on to the second best rule

which matched the input (or a prefix of the input). Multiple

REJECT's are allowed, each one finding the next best choice to

the currently active rule.

%{ int s=0;

%}

%%

she {s++; REJECT;}

he {s++}

%%

main() { yylex();

printf(“%d\n",s);

return 0; }

Exam

ple

Lex Variables

yyin Of the type FILE*. This points to the current file

being parsed by the lexer.

yyout Of the type FILE*. This points to the location where

the output of the lexer will be written. By default,

both yyin and yyout point to standard input and

output.

yytext The text of the matched pattern is stored in this

variable (char*).

yyleng Gives the length of the matched pattern.

yylineno Provides current line number

Lex Functions

yylex() The function that starts the analysis.

yywrap() In the C program generated by the lex

specification file, when yylex() finished with the

first file, it calls yywrap(), which opens the next

file, and yylex() continues. When yywrap() has

exhausted all the command line arguments, it

returns 1, and yylex() returns with value 0.

unput(c) puts the character c back onto the input stream.

It will be the next character scanned.

input() reads the next character from the input stream

Lex functions- yyless(n)

Returns all but the first n characters of the current token back

to the input stream, where they will be rescanned when the

scanner looks for the next match.

yytext and yyleng are adjusted appropriately

An argument of 0 to yyless will cause the entire current input

string to be scanned again.

For example, on the input "foobar" the following will write out

"foobarbar“

%% foobar ECHO; yyless(3);

[a-z]+ ECHO;

Lex functions- yymore()

yymore() tells the scanner that the next time it matches a

rule, the corresponding token should be appended onto the

current value of yytext rather than replacing it.

For example, given the input "mega-kludge" the following

will write "mega-mega- kludge" to the output

%% mega- ECHO; yymore();

kludge ECHO;

First "mega-" is matched and echoed to the output. Then

"kludge" is matched, but the previous "mega-" is still

hanging around at the beginning of yytext so the ECHO

for the "kludge" rule will actually write "mega-kludge".

Start Conditions

Flex provides a mechanism for conditionally activating rules.

Any rule whose pattern is prefixed with "<sc>" will only be active

when the scanner is in the start condition named "sc".

<STRING>[^"]* { /* eat up the string body ... */ ... }

will be active only when the scanner is in the "STRING" start

condition

<INITIAL,STRING,QUOTE>\. { /* handle an escape ... */ ... }

will be active only when the current start condition is either

"INITIAL", "STRING", or "QUOTE".

A start condition is activated using the BEGIN action.

Until the next BEGIN action is executed, rules with the given start

condition will be active and others are inactive.

Start conditions declared using %s are inclusive, then rules with

no start conditions at all will also be active.

Start conditions declared using %x are inclusive, then only rules

qualified with the start condition will be active.

Exclusive start conditions make it easy to specify "mini-scanners"

which scan portions of the input that are syntactically different

from the rest (e.g., comments).

Begin

BEGIN(0) returns to the original state where only the rules

with no start conditions are active.

BEGIN(INITIAL) is equivalent to BEGIN(0).

int enter_special;

%x SPECIAL

%%

if ( enter_special )

BEGIN(SPECIAL);

<SPECIAL>[a-z] {printf(“letter”);} %%

Exam

ple %s TABL REC DATA

%%

<INITIAL>"<table>" BEGIN TABL;

<TABL>"<\/table>" BEGIN

INITIAL;

<TABL><tr> BEGIN REC;

<REC><\/tr> BEGIN TABL;

<REC><td> BEGIN DATA;

<DATA><\/td> BEGIN REC;

<DATA>. ECHO;

<DATA,REC,TABL>[ \t\n] ;

%%

main()

{

yylex();

}

Line no in text file%{

/*

*/

int lineno=1;

%}

line .*\n

%%

{line} { printf("%5d %s",lineno++,yytext); }

%%

main(int argc, char*argv[])

{

extern FILE *yyin;

yyin=fopen(argv[1],"r");

yylex();

return 0;

}

A Lex program that finds a word from the given file and replaces it

with other word.

%{#include<stdio.h> #include<string.h> FILE *ff,*fr; char p[20],q[20],r[20],fname[20]; %} word [a-zA-Z]+%% {word} {

if(strcmp(p,yytext)==0) fprintf(fr,q); else fprintf(fr,yytext); }

{eol} {fprintf(fr,yytext);} . {fprintf(fr,yytext);} %% int main(int argc,char *argv[]) { strcpy(fname,argv[1]); strcpy(p,argv[2]); strcpy(q,argv[3]); ff=fopen(fname,"r+"); fr=fopen("rep.txt","w+"); yyin=ff; yylex(); return(0); }

Replace Arabic Nos with Roman%{#include<stdio.h> #include<ctype.h> FILE *ff,*fr; char fname[20];int z;%}%%1 {fprintf(fr,"I ");}2|3|4 { z=atoi(yytext);

while(z!=0){

{fprintf(fr,"I "); z--;}

}

5 {fprintf(fr,"V ");}6|7|8 { z=atoi(yytext);

{fprintf(fr,"V ");}while(z!=5){{fprintf(fr,"I "); z--;}

}9 {fprintf(fr,"IX ");}10 {fprintf(fr,"X ");}. {fprintf(fr,yytext);}%%int main(int argc,char *argv[]) { strcpy(fname,argv[1]); ff=fopen(fname,"r+"); fr=fopen("rep.txt","w+"); yyin=ff; yylex(); return(0); }

Date Format Change%{#include<stdio.h> #include<string.h> FILE *ff,*fr; char fname[20];int z;%}digit [0-9]%%{digit}{digit}"\"{digit}{digit}"\" {yymore();}[5-9][1-9] {fprintf(fr,"19"); fprintf(fr,yytext);}[0-5]{digit} {fprintf(fr,"20"); fprintf(fr,yytext);}

%%int main(int argc,char *argv[]) { strcpy(fname,argv[1]); ff=fopen(fname,"r+"); fr=fopen("rep.txt","w+"); yyin=ff; yylex(); return(0); }

The following lex specification file is used to generate a C

program which is used to generate a html file from a data

file.

A0 = "Name"

B0 = "SSN"

C0 = "HW1"

D0 = "HW2”

…

Name SSN HW1 HW2

Scott Smith 123445678 82 44.2

Sam Sneed 999887777 92 84

%%^A0 {printf("<table>\n<tr>\n");}^A[09]* {printf("</tr>\n<tr>\n");}^[BZ][09]* ;[09]* {printf("<td> "); ECHO; printf("</td>\n");}[09]*"."[09]* {printf("<td> "); ECHO; printf("</td>\n");}\"[^"]*\" {printf("<td> "); yytext[yyleng1]= ' ';

printf("%s </td>\n",yytext+1);}= ;[ \n] ;. ECHO;%%main() {printf("<html>\n");yylex();printf("</tr>\n</table>\n</html>\n");}

This lex specification file is used to generate a C program

which counts how many times a given alphabet is next to

another alphabet in the given input

%{#include<stdlib.h>int F[26][26];%}%%[AZaz][AZaz]{F[toupper(yytext[0])65][toupper(yytext[1])65]++;REJECT;}.|\n ;%%

main(int argc, char*argv[]){extern FILE *yyin;int i,j;for(i=0;i<26;i++)for(j=0;j<26;j++)F[i][j]=0;yyin=fopen(argv[1],"r");yylex();for(i=0;i<26;i++){for(j=0;j<26;j++) printf("%d", F[i][j]);printf("\n");}return 0;}

%{#include <stdio.h>#include <errno.h>int file_num;int file_num_max;char **files;extern int errno;%}%%(ftp|http):\/\/[^ \n<>"]* printf("%s\n",yytext);.|\n ;%%int main(int argc, char *argv[]) {file_num=1;file_num_max = argc;files = argv;if ( argc > 1 ) {if ( (yyin = fopen(argv[file_num],"r")) ==0 ) {perror(argv[file_num]);exit(1);

Yywrap() Example

}}while( yylex() );return 0;}int yywrap() {fclose(yyin);if ( ++file_num < file_num_max ) {if ( (yyin = fopen(files[file_num],"r")) ==0 ) {perror(files[file_num]);exit(1);}return 0;} else {return 1;

A lex-based squid redirector %x SKIP%x COPY%option always-interactive%%"http://www.debian.org/" |"http://www.yahoo.com/" {

yytext[yyleng-1] = '\0';printf("%s.au/",yytext);BEGIN COPY;}

"http://"(www.)?"altavista.digital.com/" {printf("http://www.altavista.yellowpages.com.au/");BEGIN COPY;}

"ftp://sunsite.anu.edu.au/pub/linux/" |"ftp://sunsite.unc.edu/pub/Linux/" |

"ftp://ftp.funet.fi/pub/Linux/" {printf("ftp://ftp.monash.edu.au/pub/linux/");

BEGIN COPY;}

"http://weather/" {printf("http://www.bom.gov.au/");BEGIN COPY;}

"http://bypass." { printf("http://"); BEGIN COPY; }"ftp://bypass." { printf("ftp://"); BEGIN COPY; }<COPY>[^ \n] ECHO;<COPY>" " BEGIN SKIP;. BEGIN SKIP;<SKIP>. ;<*>\n { putchar('\n'); fflush(stdout); BEGIN 0; }%%

Write a LEX program to count the number of palindrome string in a given file and write them into a separate file.

Write a LEX program to read 3 Roman numbers and output its equivalent numbers and sum in decimal.

Ex: Input: VI IX L Output: 6 + 9 + 50 = 65

Write a LEX program to read an expression in postfix form and evaluate the value of the expression

Ex: 342+* is 18

Write a lex program to read a text file and reverse all words that begin with letter ‘a’ and end with letter ‘d’. Display the longest of such words.

Write a LEX program to count number of even and odd numbers. Output the odd numbers to odd.txt and even numbers to even.txt files. Also display the largest odd and even numbers

Write a lex program to read a text file and remove all words that begin with letter ‘a’ and end with letter‘d’. Write the remaining words to a separate file.

Write a lex program to read a text file and do the following

Add 1 to all integers.Add 0.5 to all fractional numbers.Convert all words to uppercase

words Delete all alphanumeric wordsWrite resulting content to a

separate file.

Encryption and decryption

Write a Lex program that converts a file to "Pig latin."

Specifically, assume the file is a sequence of words (groups of letters) separated by whitespace. Every time you encounter a word:

1. If the first letter is a consonant, move it to the end of the word and then add ay.

2. If the first letter is a vowel, just add ay to the end of the word.

All nonletters are copied intact to the output.

April 8, 2023 42

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