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c# : Regex issue
[quoted text, click to view] > I am trying to do a pretty simple pattern match using regex. > > The pattern is ^(?:(?<Item>.*?)@:@)*$. > > This should return a match for test123@:@ but does not, instead it > never returns when I call Regex.Match, I have to kill the thread. > > The code is below, The Regulator (some will know it) returns the > correct results, I cannot work out why. > > RegexOptions options = RegexOptions.IgnoreCase | > RegexOptions.IgnorePatternWhitespace | RegexOptions.Multiline; Regex > matcher = new Regex(@"^(?:(?<Item>.*?)@:@)*$", options); Match match = > matcher.Match(@"test123@:@"); //Never returns from here. > > I have tried this on two machines, it just doesnt work. > > TIA > > JB > ^(?:(?<Item>.*?)@:@)*$ I'd giess that the problem lies in the last *, but I'm unsure why it would hang the parser. I've seen these kinds of problems before where a lot of backtracking is possible. a better pattern would be: ^(?<item>.*)@:@$ The extra ? after the first .* isn't really needed, unless there are more than one @:@ signs in your text at the end of a line (which I doubt). But as you've not sent us the text you're trying to extract your pattern from I'd have to guess that. Another pattern that would probably work would be: ^(?<item>[^@]*)@:@$ but in there I assume that @ does not precede the @:@ at all. I hope this helps. If not, please add some sample input. Another possible problem might be that your input is simply too large. The engine isn't very good at reading multiple megabytes of text at once. As you're looking at the lines one by one, you could use that to your advantage and load a couple of lines, try to match and load the next couple of lines. A StringReader would be ideal for that purpose. -- Jesse Houwing jesse.houwing at sogeti.nl
I am trying to do a pretty simple pattern match using regex.
The pattern is ^(?:(?<Item>.*?)@:@)*$. This should return a match for test123@:@ but does not, instead it never returns when I call Regex.Match, I have to kill the thread. The code is below, The Regulator (some will know it) returns the correct results, I cannot work out why. RegexOptions options = RegexOptions.IgnoreCase | RegexOptions.IgnorePatternWhitespace | RegexOptions.Multiline; Regex matcher = new Regex(@"^(?:(?<Item>.*?)@:@)*$", options); Match match = matcher.Match(@"test123@:@"); //Never returns from here. I have tried this on two machines, it just doesnt work. TIA JB
Hello John,
[quoted text, click to view] > Jesse Houwing wrote: > >> Hello John, >> >>> I am trying to do a pretty simple pattern match using regex. >>> >>> The pattern is ^(?:(?<Item>.*?)@:@)*$. >>> > <...> > >> ^(?:(?<Item>.*?)@:@)*$ >> >> I'd giess that the problem lies in the last *, but I'm unsure why it >> would hang the parser. I've seen these kinds of problems before where >> a lot of backtracking is possible. >> >> a better pattern would be: ^(?<item>.*)@:@$ >> > Hi Jesse, thanks for the reply. > I need the Group(Named Group())repetition because I am trying to match > a@:@b@:@c@:@d@:@e@:@f@:@g@:@ > 1@:@2@:@3@:@4@:@ > etc.. (a..b..c are not the real values, it is actually more > characters, but the effect should be the same. > >> The extra ? after the first .* isn't really needed, unless there are >> more than one @:@ signs in your text at the end of a line (which I >> doubt). But as you've not sent us the text you're trying to extract >> your pattern from I'd have to guess that. >> > Hmm, it is necessary in the regulator, and should be by my > understanding, to make it non-greedy. > >> Another pattern that would probably work would be: >> >> ^(?<item>[^@]*)@:@$ but in there I assume that @ does not precede the >> @:@ at all. >> >> I hope this helps. If not, please add some sample input. >> > See above. > >> Another possible problem might be that your input is simply too >> large. The engine isn't very good at reading multiple megabytes of >> text at once. As you're looking at the lines one by one, you could >> use that to your advantage and load a couple of lines, try to match >> and load the next couple of lines. A StringReader would be ideal for >> that purpose. >> > Yep, I thought of that too, I have tried it with a single line, half a > line, and a single possible match, all hang the parser. > > A funny thing though, originally when I was experimenting with less > than a line, it would work for a bit and the stop at a certain point, > now it wont even match the 1 candidate. Given your input, try the following: (?<item>[^@]+)@:@ Or even (?<item>(?:(?!@:@).)*)@:@ No ^ or $ anywhere just this pattern. It will match single occurances each time. You should be able to loop through the results with Match m = matcher.Match(text); while(m.Success) { // Do stuff m = m.NextMatch(); } You can use the match's start and end position to verify the input is correct, instead of relying on the pattern to match the whole string. This is probably much faster, and because each time you apply the pattern only a short part fo the string needs to be matched, it shouldn't make the engine hang. Another option that should be even easier to use I'd guess, would be Regex.Split on "@:@". That should result in an array of the other values. But then again, be careful with the length of the input. -- Jesse Houwing jesse.houwing at sogeti.nl
[quoted text, click to view]
Jesse Houwing wrote: <...><...> > Given your input, try the following: > > (?<item>[^@]+)@:@ > > Or even > > (?<item>(?:(?!@:@).)*)@:@ > > No ^ or $ anywhere just this pattern. It will match single occurances > each time. Thanks Jesse, I realize that there are plenty of other ways to parse this input, but I cannot figure out why it is failing when it works fine in regulator. Cheers,
[quoted text, click to view]
Jesse Houwing wrote: > Hello John, > >> I am trying to do a pretty simple pattern match using regex. >> >> The pattern is ^(?:(?<Item>.*?)@:@)*$. >> <...> > > ^(?:(?<Item>.*?)@:@)*$ > > I'd giess that the problem lies in the last *, but I'm unsure why it > would hang the parser. I've seen these kinds of problems before where a > lot of backtracking is possible. > > a better pattern would be: ^(?<item>.*)@:@$ Hi Jesse, thanks for the reply. I need the Group(Named Group())repetition because I am trying to match a@:@b@:@c@:@d@:@e@:@f@:@g@:@ 1@:@2@:@3@:@4@:@ etc.. (a..b..c are not the real values, it is actually more characters, but the effect should be the same. [quoted text, click to view] > > The extra ? after the first .* isn't really needed, unless there are > more than one @:@ signs in your text at the end of a line (which I > doubt). But as you've not sent us the text you're trying to extract > your pattern from I'd have to guess that. > Hmm, it is necessary in the regulator, and should be by my understanding, to make it non-greedy. [quoted text, click to view] > Another pattern that would probably work would be: > > ^(?<item>[^@]*)@:@$ but in there I assume that @ does not precede the > @:@ at all. > > I hope this helps. If not, please add some sample input. > See above. [quoted text, click to view] > Another possible problem might be that your input is simply too large. > The engine isn't very good at reading multiple megabytes of text at > once. As you're looking at the lines one by one, you could use that to > your advantage and load a couple of lines, try to match and load the > next couple of lines. A StringReader would be ideal for that purpose. > Yep, I thought of that too, I have tried it with a single line, half a line, and a single possible match, all hang the parser. A funny thing though, originally when I was experimenting with less than a line, it would work for a bit and the stop at a certain point, now it wont even match the 1 candidate. TIA
[quoted text, click to view]
In article <46ff8fec$0$15276$afc38c87@news.optusnet.com.au>, John B wrote: > This should return a match for test123@:@ but does not, instead it never > returns when I call Regex.Match, I have to kill the thread. It sounds as if you are running your code under the debugger. I had what sounds like the same problem a few weeks ago involving regular expressions that worked fine using a regex test tool (in my case Expresso). I noticed that if I set the breakpoint on the line after the match, the breakpoint was reached. Eventually, I discovered that if the "Autos" window wasn't open, Visual Studio was able to step though the code including the Match line. I suspect that Visual Studio has some problems with some of the objects in the Match/Regular expression objects. This might be because the VS debugger actually uses an object's ToString(format,format Provider) to display values. If the object's ToString() routine takes an exception, it must be very difficult for the debugger to handle. Any window that shows an object's value might have the same problem. Hope this helps. Mike
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