Regular Expressions in Java SE 1: Overview

regular expression

concept

• Definition: an expression that conforms to certain rules
• Role: used to specifically manipulate strings

• Features: used for some specific symbols to represent the operation of the code, which simplifies the long program code

• Benefit: Can simplify complex operations on strings

• Disadvantages: The more symbol definitions, the longer the regularity and the worse the readability

key class

• String
• Pattern: Regular class object
• Matcher: An engine class that performs matching operations on character sequences by interpreting Patterns. (matcher)

Specific operation function

match: Returns a boolean value that matches the rule

1 String:
    boolean string.matches(String regex)
2 Pattern + Matcher:
    Pattern pattern.compile(String regex)
    Matcher pattern.matcher(String regex) 
    boolean matcher.matches()
3 Pattern
    boolean b = Pattern.matches(String regex, String string)

Cut: return a string that does not match the rules

1 String:String[] split(String regex)
2 String:String[] split(String regex, int limit)

> 为了让规则的结果被重用，可以让规则封装成一个组，用()完成。组的出现都有编号：
    从1开始想要使用已有的组可以通过格式：\ + 组序号
    Eg:以叠词作为分隔符号，切隔字符串:(.)\1+
    Eg:     (X(Y(Z)))(M)
        组号：1 2 3    4   

replace: returns the string replaced by the rule

1 String:String replaceAll(String regex, String replacement):regex-\n
2 String:String replaceFirst(String regex, String replacement):replacement-$n
3 Pattern + Matcher:
    Pattern pattern.compile(String regex)
    Matcher pattern.matcher(String regex) 
    -------------------------------------
    String matcher.quoteReplacement(String s)
    String matcher.replaceAll(String replacement)
    String replaceFirst(String replacement) 

Get: Get the substring of the rule in the string

将正则表达式封装成对象；让正则表达式和要操作的字符串相关联；关联后，获取正则匹配引擎；通过引擎对符合规则的子串进行操作，如：取出
1   //step1:将规则封装成对象
    Pattern pattern = Pattern.compile(regex);
    
    //step2：让正则对象和要提取的字符串相关联,获取匹配(器)引擎对象
    Matcher matcher = pattern.matcher(string);
    
    while(matcher.find()) {//不断地利用正则引擎查找符合正则表达式的对象,找到：true；没有找到：false
        System.out.println("start:" + matcher.start() + " word:" + matcher.group() + " end:" + matcher.end());//返回当前查找到的一个子串，并返回其单词的开始start()与结束区间end()
    }