I understand your point with: 
/\b_+(\w+[^_])_+\b/

[download]
But this is not equivalent, as it requires at least 2 characters in the parens, whereas my original 
/\b_+(\w+)_+\b/

[download]
only requires 1.

Also, your regex will match lots of things in \W, instead of [a-zA-Z0-9]. Using this would be closer to what I meant: 
/\b_+(\w*[^_])_+\b/

[download]
In fact, reconsidering the original problem, I should have done something more like 
/\b_+([a-zA-Z]\w*[a-zA-Z0-9])_+\b/

[download]
because I want at least one [a-zA-Z] (although that too fails for "__x__"). Through sheer laziness, I've decided on: 
/\b_+(\w+?)_+\b/

[download]
even though this lets through pathological cases such as "_____" which I hadn't originally considered.

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

In reply to Re^2: Regex puzzle for you by QM
in thread Regex puzzle for you by QM

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