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/
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In fact, reconsidering the original problem, I should have done something more like

/\b_+([a-zA-Z]\w*[a-zA-Z0-9])_+\b/
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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/
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even though this lets through pathological cases such as "_____" which I hadn't originally considered.