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Brief introduction of regular expression

Regular expression, also known as regular representation, conventional representation. (English: Regular Expression, often abbreviated as regex, regexp or RE in code), a concept in computer science. Regular expressions use a single string to describe and match a series that conforms to a syntactic rule. In many text editors, regular expressions are often used to retrieve and replace text that matches a certain pattern.

Example 1: matches a number of 5-12 digits: ^\ d {5pr 12} $

First, we introduce two special symbols, ^ for the beginning of the string and $for the end of the string, which are used to limit the scope of the match. Then\ d is used to match a number, followed by a {}, in which the number is entered. When entering a number, such as {3}, it means that the number is matched three times; when entering two numbers, as in the example, it can be matched 5-12 times.

Example 2: match words that begin with the letter a:\ ba\ w *\ b

You can use characters to match directly in regular expressions, such as a to match a pattern with the letter a. \ b represents the edge of the character. \ w means to match all characters except spaces. Some symbols can be used in regular expressions to indicate how many times to match, where * means to match 0 times, + means to match 1 times,? Indicates a match of 0 or 1 times. So in the above example,\ w* means a followed by 0 or more characters. So the whole pattern matches characters that start with a.

Example 3: matching landline phone number:\ (? 0\ d {2} [) -]?\ d [8]

This regular expression matches a phone number such as 88886666 or 22334455. First of all, we should pay attention to it, because (? Symbols such as others have a special meaning in regular expressions, so you need to change your mind when you want to use real such symbols. The first\) in this example? It means (0 or 1 occurrence). Next is the area code, because the area code is a 3-digit number and the starting number must be 0, so using 0\ d {2} can perfectly match this requirement. Then there is the use of []. Multiple values can be placed in [], indicating that the next bit matches one of these characters. Like [) -] here? It means to match one of the left parentheses, spaces, or -, 0 or 1 times. Finally, use\ d {8} to match 8 digits to meet the requirements.

Example 4: matches the first part of the word ending in ing:\ b\ w + (? = ing\ b)

Zero-width assertions are used here, and the (? = exp) in the zero-width assertion matches the front part of the subsequent requirement. The (? = ing\ b) in this example matches the characters that end in ing.

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