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Yes, Robert, You are right. I am trying to change \\n (wchar_t '\' and wchar_t 'n') with a real new line symbols and it doesn't work. Allthought, then I am trying to find only wchar_t 'n' (or any other letter) everything is working good. The problem is that I have XML configuration file (Unicode), where all new lines are L"\\n" (!!! not '\n' !!!). I can't change that file format. Of course I can program that replace without boost::regex, but I am afraid that I'll loose my sleep afterwards...

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You're replacing each occurrence of \\n with \\n, so the string at the end should be exactly the same! :-)

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The replacement is "\n" not "\\n" so the '\' followed by 'n' in str should be replaced with a newline, right?

No. Remember the compiler gets first bite at the escape characters, so if you put a string "\\n" in your program, that gets passed to Boost.Regex as the two character string: "\n", which matches a single newline character. If you want to match a \ followed by a \n then you need a regex of "\\\n".

HTH, John.

You wanted to say: "\\\\n" ? :) Anyway, thanks a lot. This works! Didn't knew regex engine is working in that way... If he understands "\\\\n" as C++ "\\n" and "\\n" as C++ understands "\n", then how regex engine understands "\n" ...? Don't answer this letter. The problem is solved.