Perl Regex to exclude certain strings within the middle of a string - regex

Question

I have this string:
The file FILENAME has not been received
I'm trying to get the regex to match this, but not if the string is this (for example):
The file FAILNAME has not been received
I've got this regex so far:
/^(?=.*?\bThe\sfile\b)((?!FAILNAME).)*$/
But I'm unsure how to continue the expected text after the exclusion.
I hope I've explained that correctly :)
Thanks in advance!

Answer 1

I'd do it in two steps:
if ($message =~ /\AThe file (\S+) has not been received\z/ && $1 ne 'FAILNAME') {
I.e. use a regex to validate the general format and extract the filename, then check the extracted name separately.
Why cram everything into a single regex?
Speaking of which, you can actually cram arbitrary conditions into a regex. I wouldn't recommend it in this case, but:
/\AThe file (\S+) has not been received\z(?(?{ $1 eq 'FAILNAME' })(*FAIL))/
This extended pattern essentially says "if $1 equals FAILNAME, fail the match".

Answer 2

You could move the negative lookahead to after file followed by a whitespace character to assert what is directly on the right is not FAILNAME:
^The\sfile\s(?!\bFAILNAME\b).*$
Or of it can not occur in the string after The file use a quantifier:
^The\sfile\s(?!.*\bFAILNAME\b).*$
If there can not be anything before and after FAILNAME you could lookarounds:
^The\sfile\s(?!.*(?<!\S)FAILNAME(?!\S)).*$
Regex demo

Answer 3

My guess is that we wish to fail those that have FAILNAME, which your original expression seems to be working fine, and we'd then slightly modify that, which this might work:
^(?=The\sfile\s)(?!.*\s\bFAILNAME\b\s.*).*$
Demo 1
Here we are adding two spaces as an extra left and right boundaries, which if we do not wish to have those, we'd be simply excluding.
Example
use strict;
my $str = 'The file FILENAME has not been received
The file FILENAME has been received
The file FILENAME
The file AFAILNAME has not been received
FILENAME
The file FAILNAME has not been received
FAILNAME has not been received
The file FAILNAME
';
my $regex = qr/^(?=The\sfile\s)(?!.*\s\bFAILNAME\b\s.*).*$/mp;
if ( $str =~ /$regex/g ) {
print "Whole match is ${^MATCH} and its start/end positions can be obtained via \$-[0] and \$+[0]\n";
# print "Capture Group 1 is $1 and its start/end positions can be obtained via \$-[1] and \$+[1]\n";
# print "Capture Group 2 is $2 ... and so on\n";
}
# ${^POSTMATCH} and ${^PREMATCH} are also available with the use of '/p'
# Named capture groups can be called via $+{name}
RegEx Circuit
jex.im visualizes regular expressions: