#!/usr/bin/perl
use warnings;
use strict;

my %CSV = ('n' => 'n',
           't' => 't',
           'f' => 'f',
           '\\' => '\\',
          );

my %CSV1 = ('\\n' => '\\n',
           '\\t' => '\\t',
           '\\f' => '\\f',
           '\\\\' => '\\\\',
          );

my @same = qw( \\ \\\\ a ab \a \a\b \a\n \n\a \n\n abc\a abc\\a );
my @diff = qw( a\\ \a\\ abc\a\\ abc\a\\\\ \a\b\c\d\e\x\g\h\\ );

print "These are the same as your function\n";
runtests(@same);
print "These are different:\n";
runtests(@diff);

sub runtests {
  for my $in (@_) {
    print " in:    $in\n";
    $_ = $in;
    funca();
    print  "  funca:   $_\n";
    my $savea = $_;
    $_ = $in;
    funcb();
    printf "  funcb:   %-30s %s\n\n", $_, $_ eq $savea ? "SAME" : "DIF
+FERENT";
  }
}

sub funca {
  my $pos = 0;
  while (m/\\(.)/g) { $pos=pos($_);}
  s/\\(.)/exists($CSV{$1}) ? "\\$1" : "\\\\$1"/ge;
   s/^(.{$pos})\\$/$1\\\\/;
}

sub funcb {
  my $out = "";
  pos($_) = 0;
  s/\G(\\.|[^\\]+|\\$)/substr($1,0,1) eq '\\' ? $CSV1{$1} || "\\$1" : 
+$1/eg;
}
__END__
# output:
These are the same as your function
 in:    \
  funca:   \\
  funcb:   \\                             SAME

 in:    \\
  funca:   \\
  funcb:   \\                             SAME

 in:    a
  funca:   a
  funcb:   a                              SAME

 in:    ab
  funca:   ab
  funcb:   ab                             SAME

 in:    \a
  funca:   \\a
  funcb:   \\a                            SAME

 in:    \a\b
  funca:   \\a\\b
  funcb:   \\a\\b                         SAME

 in:    \a\n
  funca:   \\a\n
  funcb:   \\a\n                          SAME

 in:    \n\a
  funca:   \n\\a
  funcb:   \n\\a                          SAME

 in:    \n\n
  funca:   \n\n
  funcb:   \n\n                           SAME

 in:    abc\a
  funca:   abc\\a
  funcb:   abc\\a                         SAME

 in:    abc\a
  funca:   abc\\a
  funcb:   abc\\a                         SAME

These are different:
 in:    a\
  funca:   a\
  funcb:   a\\                            DIFFERENT

 in:    \a\
  funca:   \\a\
  funcb:   \\a\\                          DIFFERENT

 in:    abc\a\
  funca:   abc\\a\
  funcb:   abc\\a\\                       DIFFERENT

 in:    abc\a\\
  funca:   abc\\a\\\
  funcb:   abc\\a\\                       DIFFERENT

 in:    \a\b\c\d\e\x\g\h\
  funca:   \\a\\b\\c\\d\\e\\x\\g\\h\
  funcb:   \\a\\b\\c\\d\\e\\x\\g\\h\\     DIFFERENT

NT
[download]