Lookaheads and lookbehinds are not supported in POSIX regex, which is the one supported by Xbench.
Perhaps one possible workaround for what appears to be your use case could be to export the segments with Xbench as tab-delimited, do a global change of square brackets ([]) with angle brackets (<>) and load the tab-delimited file into Xbench. Then you can search or QA with the *Ignore tags setting.