Update: I think, I got this figured out before anybody saw it. lol

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I have a little exercise which might sound like homework, but I promise you it isn't! Lol And I almost got it to work, but I can't figure out how to make the last function do what it's supposed to. Update: *Got it!

So, I wanted to write four functions--one that doubles certain characters in a string and another sub that removes double characters from a string. I think I want to clarify that what I want is not this:

$S =~ tr| ||s;

I know, this would collapse multiple spaces to a single space, but that's not what I want to do. I want to do this: "AAAAAA" => "AAA" just replace each pair with a single character. OK. So, I already figured out how to do this.

But then I want to write another sub that repeats certain characters N number of times and finally a last one that finds N number of occurrences of certain characters in a string and replaces each group of characters with one character.
Find groups of 3 letter A's and replace them: "AAAAAA" => "AA"

I'm having trouble with this last sub. It just doesn't want to do what I want. Can someone please help me show me how to do this? It's called SingleCharsX()

#!/usr/bin/perl

use strict;
use warnings;

# This should print: "AABBC"
print "\n", DoubleChars("ABC", "AB");

# This should print: "LALA LLAALLAAN N"
print "\n", SingleChars("LALA LLLAAALLLLAAAAN NN", "LANG"), "\n\n";

# Okay. Now, make it more complicated...

foreach (2..7)
{
  print "\n x $_ : |",
    DoubleCharsX("ABCD", "ABXY", $_), "|";
}
print "\n\n";

foreach (2..7)
{
  print "\n x $_ : |",
    SingleCharsX("LLLLLLLL-AAAAAAAA", "ABX", $_), "|";
}

exit;

############################################
##                FUNCTIONS               ##
############################################

# This function doubles every instance of a list of characters in
# a string and returns a new string. For example, it can be used
# to double every instance of space or new line characters.
#
#    DoubleChars("Helo World!", "!lo ")  =>  "Helloo  Woorlld!!"
#    DoubleChars("Helo World!", "HHH")   =>  "HHelo World!"
#
# Usage: STRING = DoubleChars(STRING, CHARS)
#
sub DoubleChars
{
  defined $_[0] && length($_[0]) or return '';
  defined $_[1] && length($_[1]) or return $_[0];
  my $S = $_[0];
  $S =~ s/([\Q$_[1]\E]{1})/$1$1/g;
  return $S;
}

############################################
#
# This function removes certain characters that appear twice next
# to each and leaves only one instance. This can be used to undo
# the effects of the DoubleChars() function.
#
#    SingleChars(" HEELLLLLLO OO!", "OL")  =>  " HEELLLO O!"
#    SingleChars("AABBCAABBC", "AB")       =>  "ABC"
#
# Usage: STRING = DoubleChars(STRING, CHARS)
#
sub SingleChars
{
  defined $_[0] && length($_[0]) or return '';
  defined $_[1] && length($_[1]) or return $_[0];
  my $S = $_[0];
  $S =~ s/([\Q$_[1]\E]{1})\1/$1/g;
  return $S;
}

############################################

# This function doubles or triples every instance of
# a list of characters in a string and returns a new string.
# For example, it can be used to repeat every instance of
# the exclamation point. The second argument tells what
# to repeat, and the third argument tells how many times
# to repeat those characters.
#
#    DoubleCharsX("Hello World!", "!", 3)  =>  "Hello World!!!"
#
# Usage: STRING = DoubleCharsX(STRING, CHARS, [REP])
#
sub DoubleCharsX
{
  defined $_[0] && length($_[0]) or return '';
  defined $_[1] && length($_[1]) or return $_[0];
  my $REP = defined $_[2] ? int($_[2]) : 2;
  $REP >= 2 or $REP = 2;
  my $S = $_[0];
  $S =~ s/([\Q$_[1]\E]{1})/$1 x $REP/ge;
  return $S;
}

# This function doubles or triples every instance of
# a list of characters in a string and returns a new string.
# For example, it can be used to repeat every instance of
# the exclamation point. The second argument tells what
# to repeat, and the third argument tells how many times
# to repeat those characters.
#
#   SingleCharsX("Pressss a keeeeyyy!", "ey", 3) => "Pressss a keey!"
#
# Usage: STRING = SingleCharsX(STRING, CHARS, [REP])
#
sub SingleCharsX
{
  defined $_[0] && length($_[0]) or return '';
  defined $_[1] && length($_[1]) or return $_[0];
  my $REP = defined $_[2] ? int($_[2]) : 2;
  $REP >= 2 or $REP = 2;
  my $S = $_[0];
  foreach my $CHAR (split(//, $_[1]))
  {
    $S =~ s/\Q$CHAR\E{$REP}/\Q$CHAR\E/g;
  }
  return $S;
}
[download]

Update: Also, I am not sure how one should call these subs. I mean DoubleCharsX is not a very creative name. Maybe RepChars() would be better? But then how would I call SingleCharsX() ? The other thing is I don't even know what kind of title to give this thread. I'm not very good at naming these things. :P

Update: I think, everything works fine, it's not that the program doesn't do what it's supposed to, but I was expecting a different output. And since the output didn't match what I imagined it should be, I thought it was an error. I thought it should print something like this:

AABBC
LALA LLAALLAAN N


 x 2 : |AABBCD|
 x 3 : |AAABBBCD|
 x 4 : |AAAABBBBCD|
 x 5 : |AAAAABBBBBCD|
 x 6 : |AAAAAABBBBBBCD|
 x 7 : |AAAAAAABBBBBBBCD|


 x 2 : |LLLLLLLL-AAAAAA|
 x 3 : |LLLLLLLL-AAAAA|
 x 4 : |LLLLLLLL-AAAA|
 x 5 : |LLLLLLLL-AAA|
 x 6 : |LLLLLLLL-AA|
 x 7 : |LLLLLLLL-A|


AND INSTEAD IT WAS PRINTING THIS:


AABBC
LALA LLAALLAAN N


 x 2 : |AABBCD|
 x 3 : |AAABBBCD|
 x 4 : |AAAABBBBCD|
 x 5 : |AAAAABBBBBCD|
 x 6 : |AAAAAABBBBBBCD|
 x 7 : |AAAAAAABBBBBBBCD|


 x 2 : |LLLLLLLL-AAAA|
 x 3 : |LLLLLLLL-AAAA|
 x 4 : |LLLLLLLL-AA|
 x 5 : |LLLLLLLL-AAAA|
 x 6 : |LLLLLLLL-AAA|
 x 7 : |LLLLLLLL-AA|
[download]