Simple solution of regular expression

Regular expressions | Shallow solution

Regular expressions are a powerful string processing tool , You can look up strings 、 extract 、 Division 、 Replacement, etc . Is a template for matching strings .

A regular expression is a special set of text consisting of letters and symbols , It can be used to find sentences from the text that meet the format you want .

Part of the reference is from ：https://www.runoob.com/regexp/regexp-syntax.html
Regular expressions ： Online tools

1 Create regular expression

1.1 A simple example ：

^ Start position of input string for matching .
[0-9]+ Match multiple numbers , [0-9] Match a single number ,+ Match one or more .
abc$horsewithwordmotherabcandWithabcjunctiontail,$ The end of the input string for matching .

1.2 Escape sequence of non print characters ：

1.3 Special characters （ Generally, backslashes are used \ Escape character ）：

1.4 qualifiers ：

1.5 Locator ：

1.6 Predefined characters ：

1.7 Square bracket expression ：

1.8 Boundary matching character ：

1.9 Three models

• Greedy（ Greedy mode ）： The quantity indicator adopts greedy mode by default , Unless otherwise indicated . The expression of greedy mode will always match , Until it doesn't match .
• Reluctant（ Reluctantly mode ）： Suffix with a question mark （？） Express , It will only match the fewest characters . Also known as the minimum matching pattern .
• Possessive（ Possession mode ）： Suffix with a plus sign （+） Express , At present, only Java Support occupancy mode , Usually use less .
  

String str = "hello , java!";
// Regular expressions for greedy patterns 
str.replaceFirst("\\w*"," ");
// Regular expression of the reluctantly pattern 
str.replaceFirst("\\w*?"," ");

2 Using regular expressions

2.1 Use Pattern（ style ） and Matcher（ matching ） To use regular expressions .

• Pattern Objects are regular expressions After compiling stay Memory Representation in . The regular expression string must It is first compiled as Pattern object , And then again Use this Pattern Object to create the corresponding Matcher object . The state involved in performing the match remains in Matcher In the object , Multiple Matcher Objects can share the same Pattern object .

// The string is compiled into Pattern object 
Pattern p = Pattern.compile("a*b");
// utilize Pattern Object to create the corresponding Matcher object 
Matcher m = p.matcher("aaaaab");
boolean b = m.matches();// return true

• As defined above Pattern object It can be reused many times .Pattern yes Immutable classes , Available for multiple Concurrent thread safety Use .

2.2 Matcher Class provides several common methods ：

find()： Returns the destination string Does it include And Pattern Matching substring .
group()： Return to the last time with Pattern Matching Substring .
start(）： Return to the last time with Pattern The matching substring is in the destination string Starting position .
end(）： Return to the last time with Patterm The matching substring is in the destination string End position plus 1.
lookingAt(）： Returns the target string The front part And Pattern match .
matches（）： return Whole The target string is the same as Pattern match .
reset(）： Put the existing Matcher object Apply to a new character sequence .

• stay Pattern、Matcher In the introduction of class, we often see a CharSequence Interface . In short ,CharSequence A string representing various representations .

2.3 Find a phone number （ Program example ）：

public class FindGroup{
    
	public static void main(String[] args){
    
		// Use string simulation to get the web page source code from the network 
		String str = " I want to buy a book 《 insane Java The notes 》, Contact me as soon as possible 13500006666"
		+" To make friends , The phone number is 13611125565"
		+" Sell second-hand computers , Contact information 15899903312";
		// Create a Pattern object , And use it to build a Matcher object 
		// The regular expression only grabs 13x and 15X Mobile phone number of Duan 
		// Which phone numbers are actually captured , Just modify the regular expression 
		Matcher m = Pattern.compile("( (13\\d) | (15\\d) ) \\ d(8)").matcher(str);
		// All substrings that match the regular expression （ Phone number ） All output 
		while(m.find()){
    	
			System.out.println(m.group());
		}
	}
}

• Once the corresponding substring is found , Next call find() Method then looks down ; belt int Parameters Of find() Method will start from this int Search down at the index .
• start() and end() Method is used to determine the position of the substring in the target .
• matcher() and lookingAt() It's a little like that ：matcher() requirement Whole String and Pattern perfect match To return to true;lookingAt() as long as With Pattern start It will return true;
• reset() The method can combine the existing Matcher Object is applied to the new character sequence .
• Matcher Class provides the replaceAll() Method replaces all substrings in the string that match the regular expression with “xxx”;Matcher Class also provides a replaceFirst() Method , Replace only the first matching substring .
• String Class also provides replaceAll()、replaceFirst() 、split() Other methods .

2.4 Online tools

• Can not write can use , Regular expressions ： Online tools Offer help , And it also provides many common regular expressions , You can paste directly to the website .