#!/usr/bin/perl -w

#########################  The Many Faces of s///  #####################
$var = "abc
def
ghi";
$var2 = $var;

$var2 =~ s/.$/X/;    #########  "." matches the "i"!!
print "$var2\n\n";

$var2 = $var;
$var2 =~ s/.$/X/g;   #########  What if I put "g" at the end of s///?
print "$var2\n\n";   #########  The "." NEVER matches \n.

$var2 = $var;
$var2 =~ s/.$/X/m;   #########  Substitute last character in "multi-line" 
print "$var2\n\n";   #########  mode.  Should change "c" to "X".

$var2 = $var;
$var2 =~ s/.$/X/mg;  #########  Now "c", "f", and "i" should ALL become "X".
print "$var2\n\n";

$var2 = $var;
$var2 =~ s/.$/X/s;   #########  s///s is the default.
print "$var2\n\n";

$var2 = $var;
$var2 =~ s/.$/X/sg;  #########  In single line mode, $ exists in only 1 place!!
print "$var2\n\n";

$var2 = $var;
$var2 =~ s/\n/X/;    #########  Should change first newline to "X".
print "$var2\n\n";

$var2 = $var;
$var2 =~ s/\n/X/g;   #########  Change all newlines to "X" because of "g".
print "$var2\n\n";

$var2 = $var;
$var2 =~ s/\n/X/gm;  #########  No change in result because of explicit \n.
print "$var2\n\n";

$var2 = $var;
$var2 =~ s/\s/X/;    #########  Change first whitespace to "X".  Should change
print "$var2\n\n";   #########  First \n ONLY to "X".

$var2 = $var;
$var2 =~ s/\s/X/g;   #########  Change all whitespace to "X".  Should change
print "$var2\n\n";   #########  all \n to "X".

$var2 = $var;
$var2 =~ s/\s/X/gs;  #########  Same output as last print. Single-line default.
print "$var2\n\n";
 
$var = "abcde:fg:hij:k:lmn";
$var =~ s/\b([a-z]+)\b/$1X/g;   ###### \b is a word boundary.
print "$var\n"; 
#########################  Results of Execution Below  ########################

abc
def
ghX       ########  "i" changed to X as predicted.

abc
def
ghX       ########  No different from first line of output.  Why?  Because
          ########  s/// is in "single-line" mode unless you tell it
          ########  otherwise.  Also, "." CANNOT match \n in ANY mode!!

abX       ########  "c" changed to X as predicted.
def
ghi

abX       ########  Yup, "c", "f", and "i" all changed to X.
deX
ghX

abc       ########  Same outcome as first lines of output.
def
ghX

abc
def
ghX       ########  Ditto!

abcXdef   ########  Only first newline changed to "X" as predicted.
ghi

abcXdefXghi   ########  All newlines changed to "X" because of s///g.

abcXdefXghi   ########  No change in result due to explicit \n in s///mg.

abcXdef       ########  First \n changed to "X" due to explicit \n in s///.
ghi

abcXdefXghi   ########  All \n changed to "X" due to explicit \n in s///g.

abcXdefXghi   ########  Same output because s///sg is same as s///g here.

abcdeX:fgX:hijX:kX:lmnX
########################  Advanced Substitutions  #######################
#!/usr/bin/perl -w
$var = "abc123for456and789me!";
$changes = $var =~ s/\d//g;
print "$changes   $var\n";    ##  Output 1.

$var = "The file /etc/passwd is the password file.";
$var =~ s#/etc/passwd#/etc/FOOBAR#;
print "$var\n";    ##  Output 2.

$var = "10 20 30 40 50";
$var =~ s/\d+/$& ** 2/ge;   ######  $& means "pattern between the first
print "$var\n";             ######  set of //".  Output 3.

$var = "10 20 30 40 50";
$var =~ s/\d+/$& x length($&)/ge;   ##  Output 4.
print "$var\n";      

$var = "511 is an octal version of 511.";
$var =~ s/^\d+/sprintf("%o",$&)/e;
print "$var\n";   ##  Output 5.

$var = "  hEllo, I aM   JohN!";
$var =~ s/.*/clean_line($&)/e;  
print "$var\n";   ##  Output 6.

$var = "123abcdefABC";
$howmany = $var =~ s/\d{3}|[A-Z]{3}//g;
print "$var  $howmany\n";    ##  Output 7.

$var = "1234abcd123abc12345ab23";
$var =~ s/\d{3}//g;
print "$var\n";     ##  Output 8.

$var = "1234abcd123abc12345ab23";
$var =~ s/\d{3,}//g;
print "$var\n";     ##  Output 9.
###################  End of Main Routine   #######################
sub clean_line   ##  First exposure to subs!!
{
     my ($str) = @_;

     $str =~ s/^\s*(.*?)\s*$/$1/;
     $str =~ s/\s+/ /g;   ### Multi-whitespace sequences ==> one ASCII space.
     $str = lc($str); 
     return $str;
}
#####################  Program Output Below  ######################
9   abcforandme!   ## Output 1.
The file /etc/FOOBAR is the password file.  ## Output 2.

100 400 900 1600 2500      ## Output 3.
1010 2020 3030 4040 5050   ## Output 4.

777 is an octal version of 511.   ## Output 5.
hello, i am john!    ##  Output 6.
abcdef  2            ##  Output 7.
4abcdabc45ab23       ##  Output 8.
abcdabcab23          ##  Output 9.