Because you're telling the regex engine to match a line that contains the word 'is' only.

^ stands for a beginning of a line, while $ stands for the end of a line.

so unless you have a line that contains only 'is' it won't match

Your regex is better written as /\bis\b/are/g;

^ = matches at start of string (and at start of a line with /m)
$ = matches before a newline at end of string and at end of string (and before any newline with /m)
\b = at a word boundary. That is to say, it matches between something that matches \w and something that matches \W in any order. The start and end of the string count as \W for this purpose.

That is all in perlre

open(my $fh, '<', "apache.txt") or die $!;
while (<$fh>) {
    s/\bis\b/are/g;
    print;
}
[download]

perl -pe"s/\bis\b/are/g" apache.txt
[download]

$_ =~ s/^is$/are/g ;
[download]

$_ =~ s/\bis\b/are/g ;
[download]

You may be interested in My Favourite Regex Tools.

thanks. I got how to do this one. I got another questio in the process. when I code "s/^is$/are/g" , this will look for 'is; in the beginning and at the end of string. So if I have
<code> Apache is a modular server. This implies that only the most basic functionality is included in the core server. is it so ? Extended features are available through modules which can be loaded into Apache. By default, a base set of modules is included in the server at compile-time. If the server is compiled to use dynamically loaded modules, then modules can be compiled separately and added at any time using the LoadModule directive. Otherwise, Apache must be recompiled to add or remove modules. Configuration directives may be included conditional on a presence of a particular module by enclosing them in an <IfModule> block<code>
is this will not change anything in the file.
If I have
Apache is a modular server. This implies that only the most basic functionality is included in the core server. is it so ? is Extended features are available through modules which can be loaded into Apache. By default, a base set of modules is included in the server at compile-time. If the server is compiled to use dynamically loaded modules, then modules can be compiled separately and added at any time using the LoadModule directive. Otherwise, Apache must be recompiled to add or remove modules. Configuration directives may be included conditional on a presence of a particular module by enclosing them in an <IfModule> block
will this change is in the first line to are ? As its in the beginning and end of the string ?
I tried it, but it did not change anything in the file

I tried it, but it did not change anything in the file

You didn't ask for help saving strings to a file, so I left that bit to you.

perl -pe"s/\bis\b/are/g" apache.txt >apache.new
[download]

Or here's a way that renames the original file for you and saves back to the original file:

perl -i.bak -pe"s/\bis\b/are/g" apache.txt
[download]

I'm afraid I do not understand your problem. If I take ikegami's code and modify it slightly so as not to depend on the existence of "apache.txt":

while (<DATA>) {
    s/\bis\b/are/g;
    print;
}

__DATA__
Apache is a modular server. This implies that only the most basic func
+tionality
is included in the core server. is it so ?
Extended features are available through modules which can be loaded in
+to Apache.
 By default, a base set of modules is
included in the server at compile-time. If the server is compiled to u
+se dynamic
ally loaded modules, then modules can be
compiled separately and added at any time using the LoadModule directi
+ve. Otherw
ise, Apache must be recompiled to add or
remove modules. Configuration directives may be included conditional o
+n a presen
ce of a particular module by enclosing
them in an <IfModule> block.
[download]

I get the following output (where clearly "is" has been replaced by "are" throughout):

Apache are a modular server. This implies that only the most basic fun
+ctionality

are included in the core server. are it so ?
Extended features are available through modules which can be loaded in
+to Apache.

 By default, a base set of modules are
included in the server at compile-time. If the server are compiled to 
+use dynami
c
ally loaded modules, then modules can be
compiled separately and added at any time using the LoadModule directi
+ve. Otherw

ise, Apache must be recompiled to add or
remove modules. Configuration directives may be included conditional o
+n a presen

ce of a particular module by enclosing
them in an <IfModule> block.
[download]

How does this differ from your desired output?

HTH,

planetscape

I tried it, but it did not change anything in the file

You didn't ask for help saving strings to a file, so I left that bit to you.

perl -pe"s/\bis\b/are/g" apache.txt >apache.new
[download]

Or here's a way that renames the original file for you and saves back to the original file:

perl -i.bak -pe"s/\bis\b/are/g" apache.txt
[download]

I tried it, but it did not change anything in the file

ikegami's script the the correct way to do it. It doesn't change anything in the file, rather it opens the file handle for output only. If you want to modify the file, that's a totally different procedure and isn't pertinent to the question that you asked about the regex.