Based on your description of the problem and sample results, it looks like you mean shortest, not longest. Otherwise the answers are more like 
1 - .....x......
2 - .x.....x......
3 - .x.....x......x
4 - x.x.....x......x

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In reply to Re: regex finding longest string containing n of $c by Errto
in thread regex finding shortest string containing n of $c by xipho

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