Question

I want to replace only the first instance if two matches returned.

example:

$sen = "The quick brown fox jump over the lazy dog, fox is quick";

how can i match only the first fox and replace it with wolf and vice versa.

Output:

The quick brown wolf jump over the lazy dog, fox is quick 

or:

The quick brown fox jump over the lazy dog, wolf is quick 

Thanks.

Answer 1

Counting matches by using global option /g, assign list of matches to an empty list to get a count (better than a temp variable). First replace is basic. Second uses a counter that increments for each match, and replaces at the proper time.

Note use of word border \b to prevent false matches, such as firefox, foxy lady, etc.

Code:

use strict;
use warnings;
use v5.10;   # only required for say, use print instead if your version is lower

my $fox  = 'fox';
my $wolf = 'wolf';
my $sen = "The quick brown fox jump over the lazy dog, fox is quick";

if ((()=$sen =~ /\b$fox\b/g) == 2) {   # counting matches 
    my $first = $sen;
    $first =~ s/\b$fox\b/$wolf/;       # replacing first
    my $second = $sen;
    my $i = 0;
    $second =~ s/\b$fox\b/ ++$i == 2 ? $wolf : $fox/eg;  # replacing second
    say for $first, $second;
}

Output:

The quick brown wolf jump over the lazy dog, fox is quick
The quick brown fox jump over the lazy dog, wolf is quick

If you wish a more reusable code, you can make a subroutine out of it.

my $second = replace_nr($sen, $fox, $wolf, 2);
...
sub replace_nr {
    my ($str, $find, $replace, $num) = @_;
    my $i = 0;
    $str =~ s/\b($find)\b/ ++$i == $num ? $replace : $find/eg;
    return $str;
}

You could then even use the sub for both substitutions:

my $first  = replace_nr($sen, $fox, $wolf, 1);
my $second = replace_nr($sen, $fox, $wolf, 2);