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Merge pull request #2378 from vsemozhetbyt/patch-9

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Fix typo, clarify wording in 9.15 (Catastrophic backtracking)
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Ilya Kantor 2020-12-13 20:16:55 +03:00 • committed by GitHub
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9-regular-expressions/15-regexp-catastrophic-backtracking/article.md
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@ -160,7 +160,7 @@ Trying each of them is exactly the reason why the search takes so long.
## Back to words and strings
The similar thing happens in our first example, when we look words by pattern `pattern:^(\w+\s?)*$` in the string `subject:An input that hangs!`.
The similar thing happens in our first example, when we look for words by pattern `pattern:^(\w+\s?)*$` in the string `subject:An input that hangs!`.
The reason is that a word can be represented as one `pattern:\w+` or many:
@ -238,7 +238,7 @@ E.g. in the regexp `pattern:(\d+)*$` it's obvious for a human, that `pattern:+`
(1234)(56789)!
```
And in the original example `pattern:^(\w+\s?)*$` we may want to forbid backtracking in `pattern:\w+`. That is: `pattern:\w+` should match a whole word, with the maximal possible length. There's no need to lower the repetitions count in `pattern:\w+`, try to split it into two words `pattern:\w+\w+` and so on.
And in the original example `pattern:^(\w+\s?)*$` we may want to forbid backtracking in `pattern:\w+`. That is: `pattern:\w+` should match a whole word, with the maximal possible length. There's no need to lower the repetitions count in `pattern:\w+` or to split it into two words `pattern:\w+\w+` and so on.
Modern regular expression engines support possessive quantifiers for that. Regular quantifiers become possessive if we add `pattern:+` after them. That is, we use `pattern:\d++` instead of `pattern:\d+` to stop `pattern:+` from backtracking.