It looks like you need a look-ahead assersion: 

$string = /a(?!b)/;

[download]

EDIT: Well, I misread the question (paid too much attention to the title). A simple /a/ and !/a.*b/ (++1008541) or /^[^a]*a+[^b]*$/ should be enough.

Sorry if my advice was wrong.

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

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