java regex stays processing but no result is thrown

Question

so im learning regex in java and was wondering why when i execute this code

String xxx = "(\\s+)?(c:/|c:\\\\|C:\\\\|C:/|c:\\|C:\\))?(\\w+(/|\\\\)?)+(/|\\\\)\\w+.[a-z]+";

String x = "C:\\Users\\esteban\\Desktop\\Java_file_testing\\file3.txt";

    if(x.matches(xxx)) {
        System.out.println("matches");
    }else {

            System.out.println("no match found ");
    }

this prints matches but when i remove the .txt is stays processing without any response, am i doing something wrong?

@Rod_Algonquin "stays processing without any response" = "`x.matches(xxx)` doesn't return" — user253751, Aug 24 '14 at 03:24
@immibis what I want to know is the expected result. Yes I am aware of it. — Rod_Algonquin, Aug 24 '14 at 03:25
expected result should be `no match found` if i remove the .txt — shep, Aug 24 '14 at 03:29
@Avinash Raj, even if i add the `\\.` instead of `.` the result is the same : it stays processing without any result when i remove the `.txt` OR just the `txt` and leave the period ( . ) — shep, Aug 24 '14 at 03:33
This question seems related: [Infinite loop in regex in java](http://stackoverflow.com/questions/4500507/infinite-loop-in-regex-in-java) — Volune, Aug 24 '14 at 03:39

Robin · Answer 1 · 2014-08-24T04:02:37.487

You stumbled upon a case of catastrophic backtracking !

When you write (\\w+(/|\\\\)?)+, you are basically introducing the (\\w+)+ pattern into your regex. This leaves the opportunity for the regex engine to match a same string in multiple ways (either using the inner or the outer +) - the number of possible paths increases exponentially, and since the engine has to try all possible ways of matching before declaring failure it takes forever to return a value.

Also, a few general comments on your regex:

c:\\| will match, literally, the string c:|
/|\\\\ is just [/\\\\]
(\s+)? is \s*
. is a wildcard ("anything but a newline") that need to be escaped
for the c/C variations, either use [cC] or make your whole regex case insensitive
when you don't need to actually capture values, using non-capturing groups (?:...) relieves the engine of some work

Taking these into account, a regex in the spirit of your first attempt could be:

\\s*(?:[cC]:[/\\\\])?(?:\\w+[/\\\\])*\\w+\\.[a-z]+

In (?:\\w+[/\\\\]), the character class [/\\\\] isn't optional any more, thus avoiding the (\\w+)+ pattern: see demo here.

For more information on catastrophic backtracking, I'd recommend the excellent (and fun !) article by Friedl on the subject on the perl journal.

Federico Piazza · Accepted Answer · 2014-08-24T03:51:51.690

1

You regex is using dot . character that matches [A-Za-z0-9_]

You have to escape the dot as:

(\\s+)?(c:/|c:\\\\|C:\\\\|C:/|c:\\|C:\\))?(\\w+(/|\\\\)?)+(/|\\\\)\\w+\\.[a-z]+
                                                          here --------^

Btw, you can shorten your regex like this:

\s*[Cc]:(?:(?:\/|\\{1,2})\w+)+\.\w+

Working demo

Remember to escape backslashes:

\\s*[Cc]:(?:(?:\\/|\\\\{1,2})\\w+)+\\.\\w+

edited Aug 24 '14 at 03:51

answered Aug 24 '14 at 03:37

Federico Piazza

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Hello. 1. I escaped the dot but the loop still keeps on taying there processing. 2.when i use this regex `\\s*c:(?:[\\/\\]{1,2}\\w+)+\\.\\w+` it throws an exception : ` Unclosed character class near index 27 \s*c:(?:[\/\]{1,2}\w+)+\.\w+ ` – shep Aug 24 '14 at 03:51
@user3710334 Have you tried with the second regex I posted? – Federico Piazza Aug 24 '14 at 03:53
@user3710334 You have to escape the regex as this: `\\s*[Cc]:(?:(?:\\/|\\\\{1,2})\\w+)+\\.\\w+` – Federico Piazza Aug 24 '14 at 03:55
ok and this doesn't throw exception but it never matches `\\s*c:(?:[\\/\\\\]{1,2}\\w+)+\\.\\w+`<< never matches `\\s*c:(?:[\\/\\]{1,2}\\w+)+\\.\\w+` << exception – shep Aug 24 '14 at 03:55
FYI `/` isn't a delimiter here, so there's no need to escape it. – Robin Aug 24 '14 at 04:08

java regex stays processing but no result is thrown

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