"As a general practice, I find it's much safer to interpolate strings ... into regexes using \Q \E ..."

As a general rule, for regexes in general, that's fine and I'd generally do the same; however, bracketed classes are different.

Take a look at "perlrecharclass: Special Characters Inside a Bracketed Character Class". I'll leave you to acquaint yourself with the full text. Here's some pertinent extracts (my emphasis added):

Most characters that are meta characters in regular expressions ... lose their special meaning and can be used inside a character class without the need to escape them.
...
Characters that may carry a special meaning inside a character class are: \ , ^ , - , [ and ] , and are discussed below.
...
A [ is not special inside a character class, unless it's the start of a POSIX character class ... It normally does not need escaping.

So, none of the characters in $delim required escaping.

Furthermore, I generally aim to thoroughly test my solutions before posting them. In this instance, I had added a temporary print statement: 

my $prefix = qr{[^$delim]*};
print "$prefix\n";

[download]

which output: 

(?^:[^([{<]*)

[download]

That's exactly the regex I wanted.

— Ken

In reply to Re^3: bracket processing by kcott
in thread bracket processing by rajaman

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