'ab' =~ /a(?!a)/
'a' =~ /a(?!a)/
[download]

'ab' =~ /a(?=[^a])/
'a' =~ /a(?=[^a])/
[download]

is there a meaningful difference between (?![a-z]) and (?=[^a-z])? is the former recommended?

Yes, they're different regular expression patterns that match different things. (?![a-z]) asserts "not followed by any of the characters from 'a' through 'z', which includes not being followed by any character." (?=[^a-z]) asserts "followed by a single character that is not any of the characters from 'a' through 'z'." The former is a negative assertion; the latter is a positive assertion.

In your case, (?![a-z]) is what you would want to use.

[PerlMonks posting tip: Enclose Perl code in <code></code> tags, even code within paragraphs.]

UPDATE: Removed color.

In your case,  (?![a-z]) is what you would want to use.

One behavioral difference between these regexes and, in the case of the OP, the reason iaw4 would (probably) want to use this regex is that it can match at the end of a string and thus emulates the behavior of the  \b assertion. (Note:  \b can also match at the start of a string.)

>perl -wMstrict -le
"my $str = 'abcd';
 for my $rx (qr{(?=[^a-z])}, qr{(?![a-z])}, qr{\b}) {
   my @offsets;
   push @offsets, $-[1] while $str =~ m{ ($rx) }xmsg;
   if (@offsets) {
     print qq{$rx matches '$str' at offset(s) @offsets};
     }
   else {
     print qq{$rx does not match '$str'};
     }
   }
"
(?-xism:(?=[^a-z])) does not match 'abcd'
(?-xism:(?![a-z])) matches 'abcd' at offset(s) 4
(?-xism:\b) matches 'abcd' at offset(s) 0 4
[download]

(?<![a-zA-Z]) emulates \b at the beginning of an alphabetic string and (?![a-zA-Z]) emulates \b at the end of an alphabetic string.

But as I explained before, it doesn't seem from iaw4's problem description that asserting a boundary match is even needed.

UPDATE: Removed color.

Please avoid casually using colors here on perlmonks, they don't play well with themes. Customizing PerlMonks CSS, Help for Display Settings

I used color here very purposefully to set apart Perl code from its surrounding explanatory text. I did this for clarity. I didn't use color "casually."

Please avoid using themes that don't "play well" with the colors in my PerlMonks posts. :)

UPDATE: Removed color and rescinded my remarks.