Backtrack 4 Windows 7
The group has now matched once. In the above example, we could easily reduce the amount of backtracking to a very low level by better specifying what we wanted.
RegexBuddy is forgiving in that it detects it's going in circles, and aborts the match attempt. Did this website just save you a trip to the bookstore? Discovering that P can't be matched still takes one step.
But between each expansion, there are more possibilities to be tried. The fields must not contain comma's. Because the group is atomic, all backtracking information is discarded and the group is now considered a single token. So the match attempt at each backtracking position will fail, causing the regex engine to backtrack linearly. Our improved regex is a perfect solution.
But that is not always possible in such a straightforward manner. Sometimes, the lazy dot is simply a symptom of a lazy programmer. The regex is declared functional, the code is shipped to the customer, and his computer explodes. It's not too hard to imagine that at this kind of exponential rate, attempting this regex on a large file with long lines could easily take forever. The dot matches the comma!
When y fails, the regex engine backtracks. If there is no token before the group, the regex must retry the entire regex at the next position in the string.
In that case, fast and furious 7 movie in english you should use atomic grouping to prevent the regex engine from backtracking. The group has one iteration it can backtrack into. These almost always lead to catastrophic backtracking.
At first sight, this regex looks like it should do the job just fine. But the atomic group made it forget all backtracking positions. The caret matches at the start of the string and the engine enters the atomic group. However, it's not always possible or easy to rewrite your regex to make everything mutually exclusive.
The reason is that all the tokens are mutually exclusive. Since all of the lazy dots are in an atomic group, the regex engines has discarded their backtracking positions. Here's a real example from a technical support case I once handled.
Then it can no longer be expanded. The group tries a second iteration.
So we need a way to tell the regex engine not to backtrack. But then, there's nothing further to backtrack to.
When nesting repetition operators, make absolutely sure that there is only one way to match the same match. Since that also fails, the engine proceeds looking all the way to the end of the file for a second closing head tag, a second closing title tag, etc. Because the entire group is one token, no backtracking can take place once the regex engine has found a match for the group. The next token is again the dot. Nested quantifiers are repeated or alternated tokens inside a group that is itself repeated or alternated.
So far, everything happened just like in the original, troublesome regular expression. Almost no backtracking is done. Since the regex engine hasn't left the atomic group yet, it is free to backtrack inside the group. In our example, the solution is to be more exact about what we want to match. If anything fails, the regex engine will backtrack through the whole regex, but it will do so linearly.
The above regex turns ugly when the y is missing from the subject string. That is, the regex engine did not cross the closing parenthesis of the atomic group. The group again has one iteration, fails the next one, and the y fails. The star is lazy, so the dot is initially skipped.
At that point, the regex engine will backtrack. But it does not give up there. But because of the atomic group, backtracking it all takes only a single step. When we've grabbed all the x's, there's no need to backtrack.
Stack overflows are particularly nasty on Windows, since they tend to make your application vanish without a trace or explanation. The regex engine is forced to announce failure immediately.
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