Very good. 

'1\b2'

[download]
I would point out the existence of (?!), which I'm told always fails, though I'm not sure I understand it.

Update 1:

By the way, in my solution I chose to accept palindromes regardless of whether they contain only alpha characters or not.
Yes. The OP was looking for multi-word palindromes, perhaps more along the lines of the "interesting to humans" variety, which seems to be what started the thread in the first place.

Update 2: After further examination, your idea could be adapted for intervening whitespace (or indeed any noise characters) if the regex engine was re-entrant (is that with or without the hyphen?). Something like: 

       (??{
           local $N;
           ($N = $^N) =~ s/\w+//g; 
           (lc $N eq reverse lc $N)
             ? '' : (?!)
        })

[download]
which might be further streamlined to 
       (??{
           local $N;
           ($N = $^N) =~ s/\w+//g; 
           (?!) if (lc $N ne reverse lc $N)
        })

[download]
I think length $^N > 1 is superfluous, as length $^N is sufficient as a test, and (.+) would always be positive anyway (or is there a zero length character that would match?)

-QM
--
Quantum Mechanics: The dreams stuff is made of

In reply to Re^3: regex at word boundary by QM
in thread regex at word boundary by mikeraz

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