regular expression

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What is RE（Regular Expression）？

Regular expressions , Also known as regular expression .( English :Regular Expression, In code it is often abbreviated as regex、regexp or RE), It's a concept of computer science . Regular expressions are often used for retrieval 、 Replace those that match a pattern ( The rules ) The text of . Regular expressions are not limited to one language , But there are subtle differences in every language . This article aims at Java Version and python edition

The role of regular expressions

Regular expression is a logical formula for string operation , It is to use some specific characters defined in advance 、 And the combination of these specific characters , Form a " Rule string ”, This “ Rule string " A filter logic used to express strings . Use special syntax to represent character classes 、 Quantity qualifier and positional relationship , Then use these special syntax and ordinary characters to represent a pattern .

Features of regular expressions

flexibility 、 Very logical and functional ;
Complex control of strings can be achieved quickly and in a very simple way
For new contacts , It's more obscure

java The use of regular expressions in

stay Java The regular expression in is string type , The verified content is also String type . adopt string Class matches Method to realize the content matching verification . Such as :“ Verified content ”.matches(“ Regular expressions ”)

Regular expression syntax rules

Regular expression syntax rules :[ Content limitation ]{ Length limit }
Content limitation

When defining rules that qualify content , If no length limit is specified , Then the default length is 1.

Single character qualification ：
[a]: Indicates that the current content must be a letter a
Range character limit ：
[a-z0-9]: The content can be a-z Any letter between or 0-9 Any number between , Regardless of priority .
Negation restriction ：
[^abc]: The content cannot be a or b or c.

Length limit

In regular expressions, pass {} To limit the length of the content .
Fixed length :{ Fixed length value }
Range length :{ Minimum length value , Maximum length value }
[a-z]{5}: Indicates that the content range is lowercase letters a To z And the length must be 5
[a-z][2,8}: Indicates that the content range is lowercase letters a To z And the length is 2 To 8 Between , contain 2 And 8
[a-z][2}: Indicates that the content range is lowercase letters a To z And the minimum length is 2, The maximum length is unlimited
[a-z]{0,2}: Indicates that the content range is lowercase letters a To z And the minimum length is o, Maximum length is 2
Length limit symbol
Length limiting symbol refers to the completion of length limiting through predefined symbols .
?: Zero or one . Equate to {0,1}
+: Once or more . Equate to {1,}
*: Zero or more times . Equate to {0,}

Predefined characters

In regular expressions, some pre-defined characters can be used to express the content restriction . The goal is to simplify the definition of content qualification

\d Matches a numeric character , Equivalent to [o-9].
\D Matches a non-numeric character , Equivalent to [^O-9].
\n Match a line break .
\r Match a carriage return .
\s Matches any whitespace characters , Including Spaces 、 tabs 、 Page breaks and so on .
\S Matches any non-whitespace characters .
\t Match a tab .
\w Match any word character that includes an underline . Equivalent to “[A-Za-z0-9_]”".
\W Match any non word character . Equivalent to “[^A-Za-z0-9_]”.

Combination definition of regular expressions

In regular expressions, multiple content and length constraints can be combined to define .
for example Must start with a letter , The minimum length is 4, Maximum length is 8." [a-z]{1}\\w{ 3,8 }"

Common regular expressions

//  mailbox  ([a-z0-9A-Z]+[-|\\.]?)+[a-z0-9A-Z]@([a-z0-9A-Z]+(-[a-z0-9A-Z]+)?\\.)+[a-zA-Z]{2,}
	// |  Express or  \\. For any character 
// IP  Address  (25[0-5]|2[0-4]\\d|[0-1]\\d{2}|[1-9]?\\d)
// URL http(s)?://([\\w-]+\\.)+[\\w-]+(/[\\w- ./?%&=]*)?
//  Id card number  (^\\d{18}$)|(^\\d{15}$