Hello, I am looking into Boost.Regex and Boost.Xpressive for implementing grep-like functionality on large files. More specifically I want to check whether a file contains a match for an expression which does not need that much context (a few KB). regex_search and BidirectionalIterators for the whole file work, but there is a complication: I want to do other operations on the file in other threads in parallel (such as calculating hashes and extracting other data). Given that this is mostly I/O bound I would like to avoid rereading portions of the file as much as possible. My idea was to implement a circular buffer which may block the workers (and reading, if data is ever read faster than analyzed). The circular buffer needs to know when old data is no longer required, though, that is regex_search no longer works on it or backtracks into it. The best idea so far is to read blocks of data and do slightly overlapped calls of regex_search on those blocks. This approach has got at least two problems: 1) Matches and context are potentially limited to the size of the overlap. 2) Semantics of the match might be changed if I choose an arbitrary cut for the overlap (e.g. ^ and $ might change because I cut the beginning or end of a line) I would not mind to specify a maximum context of a few KB to maybe 1 MB, but I do not want to change semantics. Is there a better approach? I was thinking about supplying some intelligent iterators, but Regex/Xpressive might leave copies around longer than I anticipate, such that I still cannot decide where they are and how far they may still backtrack. If that's not really possible with the boost libraries I would be interested in alternatives, too. I am already considering looking into some grep implementations. Thanks in advance! Thomas Luzat