Why does "*" cause my tiny regextester program to crash?

Alex Folland lexlexlex at gmail.com
Sun Jan 30 21:50:00 PST 2011

On 2011-01-30 21:47, Vladimir Panteleev wrote:
> On Mon, 31 Jan 2011 03:57:44 +0200, Alex Folland <lexlexlex at gmail.com>
> wrote:
>> I wrote this little program to test for regular expression matches. I
>> compiled it with in Windows with DMD 2.051 through Visual Studio 2010
>> with Visual D. It crashes if regexbuf is just the single character,
>> "*". Why? Shouldn't it match the entire string?
> "*" in regular expressions means 0 or more instances of the previous
> entity:
> http://www.regular-expressions.info/repeat.html
> It doesn't make sense at the start of an expression. ".*" is the regexp
> that matches anything[1].
> std.regex probably can't handle invalid regexps very well. Note that
> std.regex is a new module that intends to replace the older std.regexp,
> but still has some problems.

Okay, so that particular regex is invalid.  Yeah, it still shouldn't 
crash.  You're right.  How should I prevent my program from crashing 
without fixing std.regex (code I definitely don't trust myself to 
touch)?  Would the Scope statement be useful?  I still can't figure out 
exactly what it does.  I tried using scope(exit)writeln("Bad regex."); 
just before my foreach loop, but it still crashes.  I then tried 
changing "exit" to "failure", but that didn't help either; same 
behavior.  Am I using scope wrong?

>> Also, why does it match an unlimited number of times on "$" instead of
>> just once?
> Looks like another std.regex bug.

I thought it through and decided that it might not be std.regex' bug.  I 
mean, there's no way m could have an unlimited number of elements for 
foreach to loop through, right?  Actually, it probably is std.regex' 
bug.  Though, all of this doesn't really matter since nobody uses just 
"$" as a regex, since it'd match an obvious point in any input.  I bet 
Andrei would still be irked by it if he knew though.

>> My debug build is here: http://lex.clansfx.co.uk/projects/regextester.exe
> A note for the future: compiled executables aren't very useful when
> source is available, especially considering many people here don't use
> Windows.


> [1]: A dot in a regular expression may not match newlines, depending on
> the implementation and search options.

Thanks for the extra info.  :)

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