Regular expressions around-view assertion preview: How to correctly use forward and reverse preview?

Regular expressions around, assertions and pre-checks: In-depth understanding of forward and reverse pre-checks

This article explores the usage of Lookaround, assertions and pre-checks in regular expressions in in-depth manner, and clarifies some common misunderstandings. We will use "judgment strings that do not start with baidu or end with com " as an example to analyze the practical application and potential problems of forward negation pre-check and reverse negation pre-check.

The article once mentioned using the regular expression ^(?!baidu).*$ determines strings that do not start with "baidu", and ^.*?( determines strings that do not end with "com". However, a key question is: Why is the forward pre-seeking before the matching string and the reverse pre-seeking after the matching string? This seems to be the opposite of conventional usage, and what is the practical significance of reverse negating pre-examination?

The key to the problem lies in insufficient understanding of forward and reverse pre-examination. ^(?!baidu).*$ means to match "not followed by the beginning of baidu (more precisely, the beginning of empty string)", and then match "arbitrary string" and "end". And ^.*?( means matching "start", "arbitrary string" (non-greedy match), and "previously not the end of <code>com ". The latter actually lacks meaning, because it is impossible to be com before the end of the string, so the expression is always true.

To explain more clearly, we compare the following examples:

^(?!baidu) matches "not followed by the beginning of baidu ". It only matches the beginning of the string and performs pre-checking, without actual matching.

^( matches "start", and only pre-checks are performed and do not participate in actual matching.

Therefore, the judgment does not start with "baidu", .*$ part can be omitted and judged with the partial matching function, for example in JavaScript:

/^(?!baidu)/.test('baidu.com'); // false

/^(?!baidu)/.test('www.baidu.com'); // true

/^(

The following examples further clarify the meaning and difference between forward and reverse pre-check:

The test results of /^www.(?=baidu).com$/ and /^www.(? are both <code>false because they try to match " www. followed by baidu " and "fronted .com baidu " respectively, and www.baidu.com does not meet these conditions.

The test result of /^www.(?=baidu)baidu.com$/ is true because (?=baidu) positive positive pre-examination ensures that www. is followed by baidu .

The test result of /^www.baidu(? is <code>true , because (? reverse sure pre-check ensures that <code>.com is front of baidu .

Through these examples, we can understand the location of forward and reverse pre-checks and their role in the matching process. The key is that the pre-review itself only makes judgments and does not participate in actual string matching. Misuses usually stem from misunderstandings about the pre-check mechanism.

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