#substitution, s///
#s/searchpattern/replacementpattern

# + causes the preceding character to match at least once
$content = "hotdog";
print "$content\n";
if ($content =~ s/do+g/mydog/) 
{ 
    print "new content : $content\n"; 
}

#\s : a while space character 
#/g : stands for "global", which tells Perl to replace all matches, and not just the first one. 
$content = "a lot of    white    spaces";
print "$content\n";
if ($content =~ s/\s+//g) 
{ 
    print "new content : $content\n"; 
}

$content = "dog1, dog2, dog3";
print "$content\n";
if ($content =~ s/dog/cat/) #without g only replaces first one 
{ 
    print "new content : $content\n"; 
}

$content = "dog1, dog2, dog3";
print "$content\n";
if ($content =~ s/dog/cat/g) 
{ 
    print "new content : $content\n"; 
}

print "\n\n";
$content = "line 1 multiple
I am in the middle
lines line 3";
#useful for html document decoding
if($content =~ m/mu.*
.*nes/s) {   #/s means to treat string as single line.
	print "It is found as multiple lines ... \n";
	print $&."\n";
}