http://www.perlmonks.org?node_id=1008540


in reply to Find all strings containing "a" characters, that are not followed by "b" characters.

Try 
/a(?!b)/

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with negative lookahead.

And yes, your regexp looks rather OK, except I would drop a few \b anchors: 

/a\b|a[^b]/

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or combined: 
/a(\b|[^b])/

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Will that do?

In Section Seekers of Perl Wisdom