What are your boundary conditions (the point Corion raised). The following seems to be fine:

perl -e "print \"xaxbxxxxdexfx\" =~ /x(.)(?=x)/g"
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That will print "abxf". This may or may not be appropriate. If you can clarify what to do with multiple x's, it will be easier to come up with a match.

Cheers,
Ovid

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