Isn't this a continuation of search and replace?

my $foo = "This is a test.";
my $flag = 0;
print "$flag\t$foo\n";
$flag = ($foo =~ s/a/the/);
print "$flag\t$foo\n";
[download]

#!/usr/bin/perl
use strict;
use warnings;
my $flag=0;
while(<DATA>){
 if($flag == 0){
                if($_ =~ m/<p>/){
                $_ =~ s/<p>/<con>/;
                $flag=1;
                }
                }
print $_;

}
__DATA__
<root>
<p>This is para1</p>
<p>This is para2</p>
</root>
[download]

I have tried for the matching first <p>. But in a file how to find the last </p> tag and substitute.
Please help me on this

Its nice to use regular expression. It will solve easily. Further more info, see, "man perlre". I don't know what reason do you want to use flags.

It's not the website to get the source code, First explain, what you have tried, and if you are facing any problem, ask in this forum?.

The following will work even if you give it only a single line of input. The "flag" you mention is more properly referred to as a state variable.

#!/usr/bin/perl

use strict;
use warnings;

my $retain = "";

while (<DATA>)
{
    chomp;

    if (m{^(<p>)(.*)(</p>)$})
    {
        my ($begin, $middle, $end) = ($1, $2, $3);
        # This line is inside a <p>...</p> match.
        # Handle differently if it's the first line.

        if (! $retain)
        {
            # Found first <p> in block, replace with <con>.
            # Print beginning and middle, but retain end, in case
            # this one line is also the end of the block.
            print "<con>"."$middle";
            $retain = $end;
        }
        else
        {
            print "$retain\n";          # Finish off previous line
            print "$begin"."$middle";   # Start new line
            $retain = $end;             # retain ending
        }
    }
    else
    {
        # This line is not a <p>...</p> match,
        # so flush preserved output, if any,
        # and copy this line to the output.
        if ($retain)
        {
            print "</con>\n";      # Finish off previous line
            $retain = "";
        }
        print "$_\n";
    }
}

if ($retain)
{
    print "</con>\n";      # Finish off previous line
}

#<p>This is para2</p>
#<p>This is para3</p>

__DATA__
<root>
<p>This is para1</p>
</root>
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Input cases and appropriate output:

Case 1 Input: Single line

<p>This is para1</p>
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Case 1 Output:

<con>This is para1</con>
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Case 2 Input: Multiple lines in block

<p>This is para1</p>
<p>This is para2</p>
<p>This is para3</p>
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Case 2 Output:

<con>This is para1</p>
<p>This is para2</p>
<p>This is para3</con>
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Case 3 Input: non match lines present

<root>
<p>This is para1</p>
<p>This is para2</p>
</root>
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Case 3 Output:

<root>
<con>This is para1</p>
<p>This is para2</con>
</root>
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Case 4 Input: Multiple separate blocks (I assume you want separate blocks treated separately.)

<root>
<p>This is para1</p>
<p>This is para2</p>
</root>
<stem>
<p>This is para3</p>
</stem>
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Case 4 Output:

<root>
<con>This is para1</p>
<p>This is para2</con>
</root>
<stem>
<con>This is para3</con>
</stem>
[download]

The file contains so many lines other than <p> tags.
How to use substitute function to replace

s/<p>/<con><p>/;
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When the first

tag is found. and substitute

s/<\/p>/<\/con><\/p>/;
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for the last </p> tag in the file

Now you're changing the spec!

I gave you four cases of input above. Why don't you show me the four cases of output that you expect.

Thanks for the answer. But I need some help using substitution.
Please help me. I am new to perl. I tried my level best but no luck.
Even i flag concept did'nt work as it does'nt work for the continuous files when placed below the code.

$flag1 = 1;
$flag2 = 0;
$flag3 = 0;

s{<p>}{<con>};
s{.*\K</p>}{</con>}s;      # Faster, Perl >= 5.10
s{(.*)</p>}{$1</con>}s;    # Slower, any Perl
[download]

The above code is replacing every <p> with <con>.
I have to replace only in the first <p> and skip replacing of <con> for other <p> tag.
The replacement should work only for the first <p> tag in a file and I have to replace only in the last </p> and skip replacing of </con> for other </p> tag.
The replacement should work only for the last </p> tag in a file Please help me on this.

The above code is replacing every <p> with <con>.

No, just the first. You'd need to add /g to make it replace every instance.

You said you needed to search the file. You aren't searching every line, right?

Maybe you mean to insert a <con> tag before the first <p> and a </con> after the last </p>?
The way your spec is written, you may wind up with unmatched <p> and </p> pairs.

I would show a real example (short as you can be that demo's the problem) and your current code.

In this input below:, I want to substitute <p> tag


<root>
<id>274660P4</id>
<p>This is para1</p>
<p>This is para2</p>
<date>Wednesday, October 28, 2009</date>
</root>
[download]

The output:

<root>
<id>274660P4</id>
<con><p>This is para1</p>
<p>This is para2</p>
<p>This is para3</p>
<p>This is para4</p>
<p>This is para5</p><con>
<DD>Wednesday, October 28, 2009</DD>
</root>
[download]

Please help how to write the regular expression, to substitute for first and last p tag.

Wild stab in the dark.

#!usr/bin/perl 

use strict; 
use warnings; 

my $input = do{local $/; <DATA>};

$input =~ s|(<p>)|<con>\n$1|;
$input =~ s|(.*</p>)|$1\n</con>|s;

print $input;

__DATA__
<root>
<p>This is para1</p>
<p>This is para2</p>
<date>Wednesday, October 28, 2009</date>
</root>
[download]

<root>
<con>
<p>This is para1</p>
<p>This is para2</p>
</con>
<date>Wednesday, October 28, 2009</date>
</root>
[download]

See also replace the first tag.