use Modern::Perl;

# this is the list of sections to change
my @sections = qw/ messages secure cron maillog /;

# we put it in a hash for easy lookup
my %sections = map { $_ => 1 } @sections;

# Herein we remember where we are
my $section;

while ( my $line = <DATA> )
{
    # Remember the current section
    $section = $1 
        if ( $line =~ m| \[ monitor:// (?:  / ( [^/]+ ) )+ \] |x );
        
    # Change the index line if the current section
    # is one that interests us
    $line =~ s/ index = ([A-Z0-9]+) _ (np|pr) /**     index=LINUX_${2}
+/x #<-- you can remove the indent here once you see it works
        if defined $sections{ $section };
    
    
    print $line;
}

__DATA__
[monitor:///var/log/cron]
index=NDNW_np
sourcetype=syslog

[monitor:///var/log/maillog]
index=BSKX_np
sourcetype=syslog

[monitor:///var/log/secure]
index=NDNDK_np
sourcetype=syslog

[monitor:///var/log/spooler]
index=DNWN_np
sourcetype=syslog

[monitor:///var/log/audit/audit.log]
index=XBJB_np
sourcetype=syslog

[monitor:///var/log/messages]
index=JNJSJ_pr
sourcetype=syslog

[monitor:///var/log/cron]
index=NCJG_pr
sourcetype=syslog

[monitor:///var/log/maillog]
index=JE3K_pr
sourcetype=syslog

[monitor:///var/log/secure]
index=NLNDKN_pr
sourcetype=syslog

[monitor:///var/log/spooler]
index=EKJC_pr
sourcetype=syslog

[monitor:///var/log/audit/audit.log]
index=QKNK_pr
sourcetype=syslog
[download]

Unable to break the event line due to different log event. Need help w
+ith regex to match the following and break the event. 

[{"log_message":" 

{"log_message":"

eg. The second line has two event  , unable to break the event into si
+ngle line

[{"log_message":"*****************************************************
+*********************************************************************
+************************************************}

{"log_message":"******************************************************
+**************************************************]******************
+******{"log_message":************************************************
+********************************************]
[download]

code isn`t working it changes all the indexes to linux instead of linux_pr (or linux_np)

perl -00pe 's#^\[monitor:///var/log/(?:messages|secure|cron|maillog|sp
+ooler|audit/audit\.log)\]\n.*^index\s*=\s*\k.+(?=_(?:np|pr)\s*$)#linu
+x#ms' inputs.conf

_DATA_
[monitor:///var/log/cron]
index=NDNW_np
sourcetype=syslog
...
[download]

That's not the behavior I'm seeing. I'm getting the error message "Sequence \k... not terminated in regex", and if you correct \k to \K, the code works fine for me. Please take care that when you post questions, they are as accurate as possible. Please read How do I post a question effectively?, SSCCE, and I know what I mean. Why don't you?

Update: It appears you've edited your previous question from yesterday, Regex for replacing capture group content using perl, to add this question there as well. Please don't do this, making a new post is enough - in this case a reply in the previous thread would probably have been best, so that the question has more context. And I already pointed you to How do I change/delete my post? and "It is uncool to update a node in a way that renders replies confusing or meaningless".

Replies to Regex for replacing capture group content using perl included only how to s/_pr/windows_pr/, this time (and hir second question in the aforementioned post) it's about how to s/_pr/linux_pr/ … MacOS (s)he obviously forgot :-)