java 正则表达式备忘笔记

末蓝、 2021-09-25 07:52 275阅读 0赞

### 文章目录 ###

*   *   *  判断是否匹配指定表达式的语法
         *  通过正则表达式来截取字符串
         *   *  简单截取
             *  捕获组
             *  匹配不捕获 (?:pattern)
             *  正向预测不捕获 (?=pattern)
             *  反向预测不捕获 (?!pattern)
             *  后向引用
         *  自我练习
         *   *  获取a标签的超链接地址
             *  获取class中包含a-cs1的全部a标签的超链接地址
             *  下划线转驼峰
             *  驼峰转下划线
             *  数字转为财务格式（三位逗号分割）
             *  获取url中的参数
             *  连续重复字符去重

java中的正则表达式主要有两个作用，一个是判断字符串是否匹配正则表达式，另一个是通过正则表达式来截取字符串中的一部分。

### 判断是否匹配指定表达式的语法 ###

String str = "33as";
    		String pattern = "\\d{2}.*$";
    		System.out.println(Pattern.matches(pattern, str));

### 通过正则表达式来截取字符串 ###

#### 简单截取 ####

String str = "a112b234c543d";
    		String pattern = "\\d+";
    		
    		Pattern r = Pattern.compile(pattern);
    		
    		Matcher matcher = r.matcher(str);
    		
    		while(matcher.find())
    			System.out.println(matcher.group());

输出

112
    234
    543

#### 捕获组 ####

主要使用find和group方法

private static void common3() { 
    		//截取+分组
    		String str = "a123b456c7890d";
    		String pattern = "(\\d)\\d(\\d+)";
    		
    		Pattern r = Pattern.compile(pattern);
    		
    		Matcher matcher = r.matcher(str);
    		
    		while(matcher.find()){ 
    			System.out.println(matcher.group());
    			for(int i = 0; i < matcher.groupCount(); i++){ 
    				System.out.println(matcher.group(i + 1));
    			}
    		}
    	}

输出

123
    1
    3
    456
    4
    6
    7890
    7
    90

`group()`方法获取整个正则表达式的结果，`group(int i)`获取与i值对应下标的正则表达式中括号内的匹配值。以上面例子来说，`(\\d)\\d(\\d+)` 的第一个括号获取的是第一个数字，对应`group(0)`，第二个括号对应第三个数字及之后的数字，对应`group(1)`。

#### 匹配不捕获 (?:pattern) ####

***匹配 pattern 但不捕获该匹配的子表达式***

private static void v1(){ 
    		String str = "asdfgh";
    		String pattern = "asd(?:g|fg)";
    		pattern = "asd(?:g|fg)(?:gh|h)";
    		//asdggh|asdgh|asdfggh|asdfgh
    		
    		Pattern r = Pattern.compile(pattern);
    		
    		Matcher matcher = r.matcher(str);
    		
    		while(matcher.find()){ 
    			System.out.println(matcher.group());
    			System.out.println(matcher.groupCount());
    		}
    	}

输出

asdfgh
    0

可以看到括号中的表达式被匹配了出来，但是并没有被存储起来。

#### 正向预测不捕获 (?=pattern) ####

`windows(?=\\d{3})`表示匹配后面紧跟3个数字的windows。

private static void v3(){ 
    		String str = "windows123";
    		String pattern = "windows(?=\\d{3})";
    		
    		Pattern r = Pattern.compile(pattern);
    		
    		Matcher matcher = r.matcher(str);
    		
    		while(matcher.find()){ 
    			System.out.println(matcher.group());
    		}
    	}

#### 反向预测不捕获 (?!pattern) ####

这个反向不是指方向，而是相对正向取反。  
`linux(?!\\d{2})` 可以匹配`linux1`但不能匹配`linux12`或者`linux123`

private static void v4(){ 
    		String str = "linux1";
    		String pattern = "linux(?!\\d{2})";
    		
    		Pattern r = Pattern.compile(pattern);
    		
    		Matcher matcher = r.matcher(str);
    		
    		while(matcher.find()){ 
    			System.out.println(matcher.group());
    		}
    	}

输出

linux

#### 后向引用 ####

\\1 用于正则表达式内取值，取的是第一个分组匹配到的值  
例如匹配连续重复的两个字符

String content = "aa";
    	    String pattern = "(.)\\1";
    	    boolean isMatch = Pattern.matches(pattern, content);
    	    System.out.println(isMatch);//true

匹配是否存在连续字符

String content = "asdffjhkjkk";
    	    String pattern = ".*(.)\\1+.*";
    	    boolean isMatch = Pattern.matches(pattern, content);
    	    System.out.println(isMatch);//true

### 自我练习 ###

#### 获取a标签的超链接地址 ####

private static void test1(){ 
    		String str = "<a href='localhost:9999'></a>\r\r\n ";
    		str += "<a id='a1' href=\"https://www.baidu.com\" class=\"a-cs1\">bbb</a>";
    		
    // localhost:9999
    // https://www.baidu.com
    		
    		str = str.replace("\"", "'");
    		
    		String pattern = "<a(?:[^>]*)href='([^']*)";
    		Pattern r = Pattern.compile(pattern);
    		Matcher matcher = r.matcher(str);
    		
    		while(matcher.find()){ 
    // System.out.println(matcher.group());
    			for(int i = 0; i < matcher.groupCount(); i++){ 
    				System.out.println(matcher.group(i + 1));
    			}
    		}
    			
    	}

#### 获取class中包含a-cs1的全部a标签的超链接地址 ####

private static void test2(){ 
    		String str = "<a href='localhost:9999' class=\"a-cs2\"></a>\r\r\n ";
    		str += "<a id='a1' href=\"https://www.baidu.com\" class=\"a-cs1\">bbb</a>";
    		str += "<a id='a2' href=\"https://bbs.csdn.net/forums/Java\" class=\"a-cs1; rrwer\">c</a>";
    		
    // https://www.baidu.com
    // https://bbs.csdn.net/forums/Java
    		
    		str = str.replace("\"", "'");
    		
    		String pattern = "<a(?:[^>]*)class='[^a-cs1]*a-cs1[^a-cs1]*";
    		Pattern r = Pattern.compile(pattern);
    		Matcher matcher = r.matcher(str);
    		
    		while(matcher.find()){ 
    // System.out.println(matcher.group());
    			String temp = matcher.group();
    			
    			String pattern2 = "href='(.*?)'";
    			Pattern r2 = Pattern.compile(pattern2);
    			Matcher matcher2 = r2.matcher(temp);
    			
    			while(matcher2.find()){ 
    				System.out.println(matcher2.group(1));
    			}
    		}
    			
    	}

#### 下划线转驼峰 ####

private static void test3(){ 
    		String str = "aaa_bbb_c_ddd";//aaaBbbCDdd
    		
    		String pattern = "_[a-z]";
    		Pattern r = Pattern.compile(pattern);
    		Matcher matcher = r.matcher(str);
    		
    		StringBuffer sb = new StringBuffer();
    		
    		while(matcher.find()){ 
    			matcher.appendReplacement(sb, matcher.group().toUpperCase().replace("_", ""));
    		}
    		
    		matcher.appendTail(sb);
    		System.out.println(sb);
    	}

#### 驼峰转下划线 ####

private static void test4(){ 
    		String str = "aaaBbbCDdd";//aaa_bbb_c_ddd
    		
    		String pattern = "[A-Z]";
    		Pattern r = Pattern.compile(pattern);
    		Matcher matcher = r.matcher(str);
    		
    		StringBuffer sb = new StringBuffer();
    		
    		while(matcher.find()){ 
    			matcher.appendReplacement(sb, "_" + matcher.group().toLowerCase());
    		}
    		
    		matcher.appendTail(sb);
    		System.out.println(sb);
    	}

#### 数字转为财务格式（三位逗号分割） ####

private static void test6(){ 
    		String str = "23456789.12545";//23,456,789.13
    		//23456789 23,456,789.00
    		
    		str = new BigDecimal(str).setScale(2, BigDecimal.ROUND_HALF_UP).toString();
    		str = str.replaceAll("(\\d)(?=(\\d{3})+\\.)", "$1,");
    		
    		System.out.println(str);
    	}

#### 获取url中的参数 ####

private static void test7(){ 
    		String str = "http://aaa.bbb.com?aa=12&b1=b2b&ccc=123";
    		
    // aa = 12
    // b1 = b2b
    // ccc = 123
    		
    		String pattern = "[\\?|\\&]([^=]+)=([^&]+)";
    		Pattern r = Pattern.compile(pattern);
    		Matcher matcher = r.matcher(str);
    		
    		while(matcher.find()){ 
    			System.out.println(matcher.group(1) + " = " + matcher.group(2));
    		}
    		
    	}

#### 连续重复字符去重 ####

String str = "aaa...123321222ggg";
    		String regex = "(.)\\1+";
    		Matcher matcher = Pattern.compile(regex).matcher(str);
    		String res = matcher.replaceAll("$1");
    		System.out.println(res);//a.123212g

简单解释一下，`$1`代表第一个括号内的匹配值，Matcher.replaceAll的作用是吧每个匹配的表达式（`"(.)\\1+"`）替换为括号内的值。