Try 
/a(?!b)/

[download]
with negative lookahead.

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

/a\b|a[^b]/

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

[download]
Will that do?

In reply to Re: Find all strings containing "a" characters, that are not followed by "b" characters. by bart
in thread Find all strings containing "a" characters, that are not followed by "b" characters. by Kyshtynbai

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