Regular Expressions in Java

Regular expression:

Essentially a schema that describes a set of strings that share that schema

Regular expression syntax:

character

   x character
   \\ backslash character
   \t tab character
   \n newline character
   \r carriage return


character class:
   [abc] a, b or c (simple class)
   [^abc] any character except a, b or c (negation)
   [a-zA-Z] a to z or A to Z, both inclusive (range)

Predefined character classes:
   . Any character if it is itself.
   \d Number: [0-9]
   \ w word character: [a-zA-Z_0-9]: uppercase and lowercase letters, number character

boundary matcher:
   ^ start of line
   $ end of line
   \b end of word boundary (helloword?haha:world)

Greedy quantifier (emphasis)
   X? X, one or no
   X* X, zero or more
   X+X, one or more
   X{n} X characters occur exactly n times
   X{n,} X characters appear at least n times
   X{n, m} X characters appear at least n times, but not more than m times

Program example:

public static void main(String[] args) {
       Scanner sc = new Scanner(System.in) ;
       System.out.println("Please enter a QQ number:");
       String QQ =sc.nextLine() ;
       boolean flag = checkQQ(QQ) ; //Call the regular expression
       System.out.println(flag);
   }

   public static boolean checkQQ(String qq) {
       return qq.matches("[1-9]\\d{4,14}" ) ; // The string method boolean matches(String regex) tells if this string matches the given regular expression.
   }

Pattern class

The purpose is to create a matching pattern after compiling the regular expression.

String regex = "\\?|\\*";
Pattern pattern = Pattern.compile(regex);
String patternStr = pattern.pattern();//返回\?\*

Matcher class

Match a regular expression using the pattern information provided by the Pattern instance

Pattern pattern = Pattern.compile("\\?{2}");
Matcher matcher = pattern.matcher("??"); // Matcher matcher(String input) : Convert pattern object to matcher object

boolean matches = matcher.matches();

Two common methods in Pattern

String[] split(CharSequence input)
Splits the given input sequence around matches of this pattern.
String[] split(CharSequence input, int limit) The parameter limit is added to specify the number of segments to split by
splitting the given input sequence around matches of this pattern.