note aitap <p><del>It looks like you need a [doc://perlre#Look-Around-Assertions|look-ahead assersion]: <c> $string = /a(?!b)/; </c></del></p> <p><b>EDIT</b>: Well, I misread the question (paid too much attention to the title). A simple <c>/a/ and !/a.*b/</c> (++[1008541]) or <c>/^[^a]*a+[^b]*$/</c> should be enough.</p> <!-- Node text goes above. Div tags should contain sig only --> <div class="pmsig"><div class="pmsig-980564"> Sorry if my advice was wrong. </div></div> 1008535 1008535