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

my $f = 'chapter 1,2,3';
$f =~ s/ ([0-9,]+)/\\ {$1}/;
print "$_\n" for glob $f;
[download]

$f="chapter 1,2,3"; 
if($f=~m/chap( |[a-z])*([,0-9]+)/i) { 
        print join ", ", map { "chapter $_" } split /,/,$2; 
}
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You told us what the output needs to be in one case but not what the input is, or what it should do in the general case. I suggest that you start with this, and then modify it once you've figured out what your real requirements are:

print " chapter 1,chapter 2,chapter 3\n";

You can't expect us to read your mind.

How about the fun way :)

> perl -e\
"print join', ',map {\"Chapter $_\"} splice@{[split/\D+/,'Chap 1,2,3']
+},1"
Chapter 1, Chapter 2, Chapter 3
[download]

Cheers,
R.

OTOH, many will think this plodding, literal-minded code mad.... or at best, inelegant. So be it... in the name of a clear and explicit demo of an algorithm implemented more gracefully by others, above:

#!/usr/bin/perl
use 5.016;
use Data::Dumper;
# 1030798

my $str = "chapter 1,2,3";     # out: "chapter 1, chapter  2, chapter 
+3"
my @arr = split / /,$str;      # split to 2 element array, "chapter" &
+ "1,2,3"

my @chaptnums = split /,/,$arr[1];
for my $elem(@chaptnums) {
    print "$arr[0] $elem";      # e.g. "chapter " . num
    if (scalar $elem != (1+$#chaptnums) ) {  # not the last @chaptnums
+ element?
        print ", ";
    } else {
        print "\n";
    }
    
}

=head EXECUTION

C:>1030798.pl
chapter 1, chapter 2, chapter 3

=cut
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Again, CAUTION: The code above is NOT how to do the job; it's for instruction purposes, only.

#!/usr/bin/perl
use 5.016;
# 1030798ALT

my $str = "chapter 1,2,3";     # out: "chapter 1,chapter  2,chapter 3"
my ($chapter, @num);

if ( $str =~ /^(chapter) / ) {
    $chapter = $1;
    # say "\$chapter: |$chapter|";
}

my @chapnums = ( $str =~ /(\d[,]*)/gc);
    if ( @chapnums) {
        push @num,@chapnums;
    }

for my $num(@num) { 
    # say "DEBUG   \$#num: $#num";
    # if ( $1 != (1 + $#num) ) {
    #     print "\$chapter $num" . ",";
    # } elsif ( $1 == (1 + $#num) ){
    print "$chapter $num";
}
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output: chapter 1,chapter 2,chapter 3

Less plodding, perhaps less mad, but still a longform version of suggestions above.

virudinesh: your title and narrative don't agree about whether you want a comma after "chapter 3" -- an internal contradition that suggests carelessness on your part, and makes it difficult for the Monks to offer a definitive answer. It's not that doing it either way is hard; it's that it's hard to know what you want, and thus, what help you need.