Can you reverse the order ?

my $capture2 = $string, 9, 6, 'STRING'; 
my $capture1 = $string, 6, 2, "is stretched";
[download]

It seems to be last option

#!/usr/bin/perl
use strict;
use warnings;

my $string = 'This is string';
my $capture1 = $string, 6, 2, "is stretched"; # This will substitute a
+s expected
my $capture2 = $string, 9, 6, 'STRING'; # Now here original string wil
+l be updated and positions I have already are of no use.

__END__
Useless use of a constant (6) in void context at C:\test.pl line 6.
Useless use of a constant (2) in void context at C:\test.pl line 6.
Useless use of a constant ("is stretched") in void context at C:\test.
+pl line 6.
Useless use of a constant (9) in void context at C:\test.pl line 7.
Useless use of a constant (6) in void context at C:test.pl line 7.
Useless use of a constant ("STRING") in void context at C:\test.pl lin
+e 7.

Process completed successfully
[download]

That was typo now I have corrected it

Updated code:


#!/usr/bin/perl
use strict;
use warnings;

my $string = 'This is string';
my $capture1 = substr $string, 6, 2, "is stretched"; # This will subst
+itute as expected

# REALLY?

print $string,"\n";

#  I get:
#  "This iis stretchedstring"

# perhaps better is:
my $string2 = 'This is string';
$string2 =~ s/string/streched string/;
print "$string2\n";

#  that prints:
#  "This is streched string"

# Your second substr():

substr $string, 9, 6, 'STRING';
print $string,"\n";

# that prints:
# This iis STRINGhedstring

# I suspect that you want:
$string2 =~ s/string/STRING/;
print "$string2\n";

# that prints:
# "This is streched STRING"
[download]

Perl is wonderful with what are called "regular expressions" or regex'es. In general, use the power of the Perl language. substr() is a low level thing that can be used, but is very, very rare in Perl programs. substr() is very common in C programs because there is not a "built in regex" operator.

To summarize:

#!/usr/bin/perl
use strict;
use warnings;

my $string = "This is string";
print "$string\n";

$string =~ s/string/stretched string/;
$string =~ s/string/STRING/;

print "$string\n"; 

__END__
This is string
This is stretched STRING
[download]

As poj suggests, using reverse allows you to use the positions without any messy arithmetic. Here is an example from another thread. A similar approach can be useful when splice'ing several elements into different positions in an array.

Another example:

c:\@Work\Perl\monks>perl -wMstrict -le
"use Data::Dump qw(dd);
 ;;
 my $s = 'abXXcXXXXdefXXXXgXXhiXXXXXXXjkXX';
 print qq{$s};
 ;;
 my @off_len;
 while ($s =~ m{ X+ }xmsg) {
   push @off_len, [ $-[0], $+[0]-$-[0] ];
   }
 dd \@off_len;
 ;;
 for my $ar_ol (reverse @off_len) {
   my ($off, $len) = @$ar_ol;
   substr $s, $off, $len, 'YYYYY';
   }
 print qq{$s};
"
abXXcXXXXdefXXXXgXXhiXXXXXXXjkXX
[[2, 2], [5, 4], [12, 4], [17, 2], [21, 7], [30, 2]]

abYYYYYcYYYYYdefYYYYYgYYYYYhiYYYYYjkYYYYY
[download]

Update 1: Deleted a useless  my $len_s = length $s; statement left over from a previous iteration of the example code. Oops...

Update 2: Here's another example that uses an arbitrary replacement array:

c:\@Work\Perl\monks>perl -wMstrict -le
"use Data::Dump qw(dd);
 ;;
 my $s = 'ab THE c RAIN def IN g SPAIN hi FALLS jk MAINLY l ON m';
 print qq{$s};
 ;;
 my @off_len;
 while ($s =~ m{ [[:upper:]]+ }xmsg) {
   push @off_len, [ $-[0], $+[0]-$-[0] ];
   }
 dd \@off_len;
 ;;
 my @repl = qw(FOURSCORE AND 7 YEARS AGO OUR FATHERS XXX YYY);
 ;;
 my $least_i = $#off_len < $#repl ? $#off_len : $#repl;
 for my $rli (reverse 0 .. $least_i) {
   my ($off, $len) = @{ $off_len[$rli] };
   substr $s, $off, $len, $repl[$rli];
   }
 print qq{$s};
"
ab THE c RAIN def IN g SPAIN hi FALLS jk MAINLY l ON m
[[3, 3], [9, 4], [18, 2], [23, 5], [32, 5], [41, 6], [50, 2]]

ab FOURSCORE c AND def 7 g YEARS hi AGO jk OUR l FATHERS m
[download]

Yes it is related to that. I am trying to find out perticular tags in html for example a tags to skip them in my processing of making all url clickable. Simple I am trying to replace all urls with a href so that they become clickable. For that I am trying to skip a tags and all others with url will have a href inserted

Simple I am trying to replace all urls with a href

You mean, urls in text nodes? Someting like this?

use Modern::Perl;
use HTML::Parser;
use Regexp::Common qw( URI );

my $parser = HTML::Parser->new
(
    default_h => [sub { 
        my $something = shift;
        print $something 
    }, 'text'],
    
    text_h    => [sub { 
        my $text = shift;
        
        # Guess a bit and add a scheme to www
        # This might be bit too aggressive
        $text =~ s^ www\.^ http://www.^gi;
        
        # Replace URLs in the text by links
        $text =~ s^($RE{URI})^<a href="$1">$1</a>^g;
        
        print $text;
    }, 'text'],
);

$parser->parse( join "", <DATA>) || die $!;
                       
__DATA__
<!DOCTYPE html>
<html>
    <head><title>Test data </title></head>
    <body>
        Some URL without scheme www.perlmonks.org 
        and a plain URL with scheme http://perl.org
        and even ftp ftp://ftpserver.foo.com
        and an existing <a href="https://metacpan.org/">link</a> 
    </body>
</html>
[download]

Output:

<!DOCTYPE html>
<html>
    <head><title>Test data </title></head>
    <body>
        Some URL without scheme <a href="www.perlmonks.org">www.perlmo
+nks.org</a>
        and a plain URL with scheme <a href="http://perl.org">http://p
+erl.org</a>
        and even ftp <a href="ftp://ftpserver.foo.com">ftp://ftpserver
+.foo.com</a>
        and an existing <a href="https://metacpan.org/">link</a> 
    </body>
</html>
[download]



holli

You can lead your users to water, but alas, you cannot drown them.

Did you know that there's a tutorial for just this very topic: Change URIs in Text to HTML-Links?

You might also be interested in these other tutorials regarding the HTML-parsing aspect of your task: HTML::TokeParser Tutorial and HTML::Tree(Builder) in 6 minutes.