Hi perlmonks,

I've been successfully (?) getting into Perl but now I'm a little stuck here.

Many times I found great solutions from you guys so I decided to register and make a thread myself now.


Say I got a string 'x x x a x x x b x x x a x x x b x x x' and want to replace all 'a's with a 'b' and all 'b's with an 'a'.

What's the best way of achieving this without making it replace a previously replaced 'a' (that now is a 'b') with an 'a'?


Here are my several ways I came up with, one of them fails due to the issue described above:



#!/usr/bin/env perl

use strict;
use warnings;

my $test = 'x x x a x x x b x x x a x x x b x x x';
print "ORIGINAL: $test\n\n";


# 1 - array
my @array = split(/ /, $test);
for (@array) {
   if ($_ eq 'a') { $_ = 'b'; }
   elsif ($_ eq 'b') { $_ = 'a'; }
}
print "ARRAY: @array\n";


# 2 - s/// (FAILS)
my $s = $test;
$s =~ s/a/b/g;
$s =~ s/b/a/g;
print "s///: $s\n";


# 3 - s/// map
my $map = join(' ', map{ if (/a/) { s/a/b/; } elsif (/b/) { s/b/a/; } 
+$_ } split(/ /, $test));
print "s/// MAP: $map\n";


# 4 - substr
my $substr = $test;
for (my $i = 0; $i <= length($substr); $i++) {
   if (substr($substr, $i, 1) eq 'a') { substr($substr, $i, 1) = 'b'; 
+}
   elsif (substr($substr, $i, 1) eq 'b') { substr($substr, $i, 1) = 'a
+'; }
}
print "SUBSTR: $substr\n";

[download]



Output:
ORIGINAL: x x x a x x x b x x x a x x x b x x x

ARRAY: x x x b x x x a x x x b x x x a x x x
s///: x x x a x x x a x x x a x x x a x x x
s/// MAP: x x x b x x x a x x x b x x x a x x x
SUBSTR: x x x b x x x a x x x b x x x a x x x


Is there any way to achieve this with a single s/// operation?


Thanks for any suggestions that will all be greatly appreciated!

In reply to Multiple replacements with the exception of previously made replacements. by jthiel

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