Bug 8725 – segmentation fault with negative-lookahead in module-level regex

The following program crashes with a segmentation fault: ------------- #!/usr/bin/env rdmd import std.stdio; import std.regex; auto italic = regex( r"\* (?!\s+) (.*?) (?!\s+) \*", "gx" ); void main() { string input = "this * is* interesting, *very* interesting"; writeln( replace( input, italic, "<i>$1</i>" ) ); } -------------- If one removes the first line with (?!\s+), then the program doesn't crash. I was under the impression that this snippet of code operates under the SafeD subset and therefore shouldn't cause a segmentation fault. A thrown exception on problems or something, that I can understand. But a segfault? In other sad news, these are the first lines of D I've ever written :( ... so much for experimentation...

Oh, and the segfault goes away if I put the regex creation directly in the call, like so: writeln( replace( input, regex( r"\* (?!\s+) (.*?) (?!\s+) \*", "gx" ), "<i>$1</i>" ) );

I suspect that is a long standing bug with compile-time evaluation that compiler parses regex pattern at compile time wrongly (unlike at R-T). See also: http://d.puremagic.com/issues/show_bug.cgi?id=7810 The problem is that once D compiler sees an initialized global variable it has to const-fold it: int fact10 = factorial(10); //will compute and hardcode the value of factorial(10) then with regex ...: auto italic = regex( ... ); // *parses* and *generates* binary object for compiled regex pattern object with all the datastructures for matching it All of this *at compile time* via CTFE, see about it here (near the bottom of): http://dlang.org/function.html Though previously it only caused unexpectedly long compilation time (CTFE is slow) and in a select cases it failed with assert *during compilation*, it never segfaulted. Probably internal structure has subtle corruption that self-test failed to catch. E.g this one also works because italic regex is created at run-time: import std.stdio; import std.regex; void main() { auto italic = regex( r"\* (?!\s+) (.*?) (?!\s+) \*", "gx" ); string input = "this * is* interesting, *very* interesting"; writeln( replace( input, italic, "<i>$1</i>" ) ); } Also a tip: the second lookahead should be lookbehind! As is is it will test that \* is not a space indeed... Also both can be just \s, because \s+ matches whenever \s matches. And since you don't capture the contents of lookahead/lookbehind it'll be faster/simpler to use a single \s. About SafeD: it shouldn't segfault but the program listed is @system (as this is the default) :). Otherwise since regex is @trusted, it's my responsibilty to verfiy that it is memory safe, so blame me (or rather the compiler). To be actually in SafeD try putting @safe: at the top of your code or just tag main and all functions with @safe. AFAIK writeln in SafeD wouldn't work as it's still @system (obviously it should be safe/trusted). To be honest SafeD hasn't been addressed properly in the standard library yet.

Thanks for the explanation! WRT the regex string being faulty, I was aware of that; I was just experimenting when I encountered a segfault. Thanks for the pointer about adding @safe: at the top; too bad writeln is still @system. That kinda kills the usefulness of SafeD, doesn't it? I mean if I literally can't write a Hello World program in SafeD, then SafeD is quite far from ready. :) I've read the TDPL last week and this is my first encounter with writing real D code; all in all, the language is freaking awesome (goodbye C++) and I'm even willing to live with esoteric bugs in the compiler/libs if I can work around them. I understand that D is still a work-in-progress language. I intend to write a substantial (multi KLOC) D program as a learning experience; will report any bugs I find as I find them. Anyway, good luck fixing this. :)

Works with current git master. Must have been fixed along with the compiler bug in 7810. *** This issue has been marked as a duplicate of issue 7810 ***

Commit pushed to master at https://github.com/D-Programming-Language/phobos https://github.com/D-Programming-Language/phobos/commit/0f2947d4d1360f0a0f797279e6f13f95695e45ec bugfixes for compile-time regex fix issue 8725 fix issue 8349