String s1 = "ABC";
String s2 = "def";
String s3 = s1 + s2;

String str = "";
long t1 = System.currentTimeMillis();
for(int i=0; i<50000; i++){
    str = str + "A";
}
long t2 = System.currentTimeMillis();
System.out.pritnln(t2 - t1);

package string;

public class StringBuilderDemo03 {
    public static void main(String[] args) {
        /*
         * 測(cè)試StringBuilder的連接性能
         */
        StringBuilder buf = new StringBuilder();
        long t1 = System.currentTimeMillis();
        for (int i =0; i<50000; i++){
            buf.append("A");
        }
        long t2 = System.currentTimeMillis();
        System.out.println(t2-t1);
    }
}

//StringBuilder API 

buf-> char[]{A, C, B, 0, 0, 0, 0, 0, 0, 0, 0...0}
//           0  1  2  3  4  5
StringBuilder buf = new StringBuilder();
buf.append("A") 
   .append("A") 
   .append("A")
   .append("B") 
   .insert(1,"C")
   .delete(2,4);
Stirng str = buf.toString();

正則規(guī)則 rule = "HelloWorld"
字符串： s1 = "HelloKitty"; 
字符串： s2 = "HelloWorld";
// s1 s2 中那個(gè)字符串符合 rule 約定的規(guī)則？
boolean b1 = s1.matches(rule); //false
boolean b2 = s2.matches(rule); //true

package string;
public class RegDemo05 {
    public static void main(String[] args) {
        /*
         * 測(cè)試正則表達(dá)式
         */
        //定義正則表達(dá)式
        String rule = "HelloWorld";
        //定義被檢測(cè)的字符串
        String s1 = "HelloKitty";
        String s2 = "HelloWorld";
        //檢測(cè) s1 是否符合規(guī)則
        boolean b1 = s1.matches(rule);
        //檢測(cè) s2 是否符合規(guī)則
        boolean b2 = s2.matches(rule);
        System.out.println(b1);
        System.out.println(b2);
    }
}

[123456]

Hello[123456]

| 規(guī)則               | 正則表達(dá)式                   | 范圍     |
| ------------------ | ---------------------------- | -------- |
| 匹配 0~9 一個(gè)字符  | [0123456789]                 | [0-9]    |
| 匹配A-Z一個(gè)字符    | [ABCDEFGHIJKLMNOPQRSTUVWXYZ] | [A-Z]    |
| 匹配a-z一個(gè)字符    | ...                          | [a-z]    |
| 匹配a-zA-Z一個(gè)字符 |                              | [a-zA-Z] |

Hello[1-6]

| 規(guī)則               | 正則         | 預(yù)定義字符集 | 栗子         |
| ------------------ | ------------ | ------------ | ------------ |
| 匹配一個(gè)數(shù)字       | [0-9]        | \d           | Hello\d      |
| 匹配一個(gè)單詞字符   | [a-zA-Z0-9_] | \w           | A\w          |
| 匹配一個(gè)空白字符   |              | \s           | Hello\sWorld |
| 匹配任意一個(gè)字符   |              | .            | A.           |
| 匹配一個(gè)非數(shù)字     |              | \D           |              |
| 匹配一個(gè)非空白     |              | \S           |              |
| 匹配一個(gè)非單詞字符 |              | \W           |              |

package string;
public class RegDemo07 {
    public static void main(String[] args) {
        /*
         * 測(cè)試 用戶名規(guī)則：6個(gè)單詞字符組成
         * - \ 在java字符串中需要進(jìn)行轉(zhuǎn)義為 \\
         */
        //正則表達(dá)式:  
        String reg = "\\w\\w\\w\\w\\w\\w";
        System.out.println(reg);
        //被檢查的字符串
        String s1 = "Jerry1"; //可以通過(guò)檢查
        String s2 = "Tom-12"; //不可以通過(guò)檢查
        String s3 = "Andy";   //不可以通過(guò)檢查
        System.out.println(s1.matches(reg));
        System.out.println(s2.matches(reg));
        System.out.println(s3.matches(reg));
    }
}

\w\w\w\w\w\w  等價(jià)  \w{6}  

\. 匹配點(diǎn) 
\[ 匹配 [
\? 匹配 ?
\* 匹配 *
\+ 匹配 +
\\ 匹配 \

1. `\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}`
2. `(\d{1,3}\.)(\d{1,3}\.)(\d{1,3}\.)\d{1,3}`

3. `(\d{1,3}\.){3}\d{1,3}`

package string;

public class RegDemo11 {
    public static void main(String[] args) {
        /*
         * 檢查IP地址是否符合規(guī)則
         */
        //定義正則規(guī)則
        //String reg = "\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}";
        String reg = "\\d{1,3}(\\.\\d{1,3})(\\.\\d{1,3})(\\.\\d{1,3})";
        //String reg = "(\\d{1,3}\\.){3}\\d{1,3}"; //測(cè)試分組
        //定義被檢查的字符串
        String ip1 = "192.168.2.70";
        String ip2 = "10.0.0.20";
        String ip3 = "8.8.8.8";
        //定義錯(cuò)誤的被檢查字符串
        String ip4 = "192點(diǎn)168點(diǎn)2點(diǎn)70";
        String ip5 = "192-168-2-70";
        //檢查
        System.out.println(ip1.matches(reg));
        System.out.println(ip2.matches(reg));
        System.out.println(ip3.matches(reg));
        System.out.println(ip4.matches(reg));
        System.out.println(ip5.matches(reg));
    }
}

1. `\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}`
2. `\d{1,3}(\.\d{1,3})(\.\d{1,3})(\.\d{1,3})` 

3.  `\d{1,3}(\.\d{1,3}){3}`

(\d{1,3}\.){3}\d{1,3}  (分組){3} 分組的整體出現(xiàn)3次
\d{1,3}\.{3}\d{1,3}   \.{3} .必須出現(xiàn)2次,可以匹配 “192...168”

String str = "1, Tom, 110, tom@tedu.cn";
//             ,    ,    ,
//  arr=     "1" " Tom" " 110" " tom@tedu.cn"
String[] arr = str.split(",");
for(int i=0; i<arr.length; i++){
    System.out.println(arr[i]);
}

Scanner scanner = new Scanner(System.in);
System.out.print("請(qǐng)輸入:");
String str = scanner.nextLine();
//String str = "我天疫情又嚴(yán)重了，我去，又要做核算了。";
// str.replaceAll("正則", 替換字符串);
String s = str.replaceAll("我[去草靠艸]", "***");
System.out.println(s);