regular expression

Regular expressions ：

• Used to describe the content format of string , It is usually used to match whether a string meets the format requirements
• Syntax for regular expressions ：

1.[]: Represents a character , This character can be [] Specified in

  for example :

  [abc]: This character can be a or b or c

  [a-z]: Represents any lowercase letter

  [a-zA-Z]: Represents any letter

  [a-zA-Z0-9]: Represents any alphanumeric

  [a-zA-Z0-9_]: Indicates an underscore of any number or letter

  [^abc]: As long as the character is not a or b or c

2. Predefined characters :

  .: Represents any character , There's no scope

  \d: Represents any number , Equate to [0-9]

  \w: Represents any word character , Equate to [a-zA-Z0-9_]---- Word characters refer to letters / Numbers /_

  \s: Represents any white space character

  \D: It's not a number

  \W: Not word characters

  \S: It's not a blank character

3. quantifiers :

  ?: It means that the previous content appears 0-1 Time

    for example : [abc]? Can match :a or b or c Or write nothing

  +: Indicates that the previous content appears at least 1 Time

    for example : [abc]+ Can match :b or aaaaaaaaaa... or abcabcbabcbabcba....

          But it doesn't match : Write nothing or abcfdfsbbaqbb34bbwer...

  *: Indicates that the previous content appears any time (0- many times )--- Match content with + Agreement , Just don't write once

    for example : [abc]* Can match :b or aaaaaaaaaa... or abcabcba.... Or write nothing

          But it doesn't match :abcfdfsbbaqbb34bbwer...

  {n}: It means that the previous content appears n Time

      for example : [abc]{3} Can match :aaa or bbb or aab or abc or bbc

           But it doesn't match : aaaa or aad

  {n,m}: Indicates that the previous content appears at least n Most times m Time

        for example : [abc]{3,5} Can match :aaa or   abcab perhaps abcc

        But it doesn't match :aaaaaa or aabbd

  {n,}: It means that the previous content appears n More than once ( contain n Time )

       for example : [abc]{3,} Can match :aaa or aaaaa.... or abcbabbcbabcba....

            But it doesn't match :aa or abbdaw...

4.() For grouping , Is to see the contents in parentheses as a whole     

  for example : (abc){3} Express abc As a whole 3 Time . Can match abcabcabc

        But it doesn't match aaa or abcabc

        (abc|def){3} Express abc or def As a whole 3 Time .

        Can match : abcabcabc or defdefdef or abcdefabc

        But it doesn't match abcdef or abcdfbdef

 

String Support methods related to regular expressions ：

• matches()： Use the given regular expression (regex) Verify whether the format of the current string meets the requirements , If yes, return true, Otherwise return to false

                 

public class MatchesDemo {

    public static void main(String[] args) {

        String email = "[email protected]";

        String regex = "[a-zA-Z0-9_][email protected][a-zA-Z0-9]+(\\.[a-zA-Z]+)+";

        // Use regex matching email Whether the format requirements are met 

        boolean match = email.matches(regex);

        if(match){

            System.out.println(" Is the correct email format ");

        }else{

            System.out.println(" It's not the right email format ");

        }

    }

}

 

• replaceAll()： Will satisfy the regular expression in the current string (regex) Replace the part of with the given string (s)

public class ReplaceAllDemo {

    public static void main(String[] args) {

        String line = "abc123def456ghi78";

        line = line.replaceAll("[0-9]+","#NUMBER#");

        System.out.println(line);

    }

}

 

• split()： Split the current string according to the part that satisfies the regular expression , Will split out with String[] Form to return

public class SplitDemo {

    public static void main(String[] args) {

        String line = "abc123def456ghi";

        String[] data = line.split("[0-9]+"); // Split by number ( The numbers are removed )

        System.out.println(Arrays.toString(data)); // take data The array is converted to a string and output 

        line = "123.456.78";

        data = line.split("\\."); // Press . Demolition (. It's gone )

        System.out.println(Arrays.toString(data));



        // At the beginning, it is a separable item (.), Then the array number 1 Elements are empty strings ""

        // If two in a row ( More than two ) Splittable items , Then an empty string will also be pulled out in the middle ""

        // If there are multiple separable items at the end , Then the empty string is ignored 

        line = ".123.456..78.......";

        data = line.split("\\."); // Press . Demolition (. It's gone )

        System.out.println(Arrays.toString(data));

    }

}