[Java] Summary of regular expressions

List

Phone / Zip Code Matching Sample

//Sample 1 Phone/Zip code match
import java.util.*;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Main {
 public static void main(String[] args) {
    //Character string to judge
    String str = "012-345-6789";
    //Generate a pattern to judge
    Pattern p = Pattern.compile("\\d{2,4}-\\d{2,4}-\\d{4}"); //phone number
    //Pattern p = Pattern.compile("^\\d{3}-\\d{4}$"); //Postal code
    Matcher m = p.matcher(str);

    System.out.println(m.find()); //true
    }
}

Forward / backward / partial match sample

//Sample 2 forward/Rear/Partial Match
public class Main {
 public static void main(String[] args) {
    //Character string to judge
    String str = "000012-345-6789";
    //Generate a pattern to judge
    //Pattern p = Pattern.compile("\\d{2,4}-\\d{2,4}-\\d{4}.*"); //Phone number prefix match
    //Pattern p = Pattern.compile(".*\\d{2,4}-\\d{2,4}-\\d{4}"); //Phone number suffix match
    Pattern p = Pattern.compile(".*\\d{2,4}-\\d{2,4}-\\d{4}.*"); //Phone number partial match
    Matcher m = p.matcher(str);

    System.out.println(m.find()); //true
    }
}

Extract the matched string

• Prepare Pattern object with compile method of Pattern class
• Create Macher object to search for strings with matcher method
• Repeat matching with while (match.find ()) until the find method becomes false, and get the start position (start), end position (end), and match character string (group) of the string.
• group [0]: entire matched string
• group [1]: First element of the submatch string

import java.util.*;
import java.util.regex.Pattern;

public class Main {
  public static void main(String[] args) {
    var str = "Mobile is 0123-99-It's 0000. Home is 000-123-4567 and so on.";
    var ptn = Pattern.compile("(\\d{2,4})-(\\d{2,4})-(\\d{4})");
    var match = ptn.matcher(str);
    while (match.find()) {
      System.out.println("Starting position:" + match.start());
      System.out.println("End position:" + match.end());
      System.out.println("Matching string:" + match.group());
      System.out.println("Area code:" + match.group(1));
      System.out.println("City code:" + match.group(2));
      System.out.println("Subscriber number:" + match.group(3));
      System.out.println("-----");
    }
  }
}

Behavior control during matching

• You can change the behavior when instantiating the Pattern class with the match flag.

CASE_INSENSITIVE

• Case insensitive

import java.util.*;
import java.util.regex.Pattern;

public class Main {
  public static void main(String[] args) {
    var str = "For work is [email protected] is com. NEKO for private [email protected] is com.";
    var ptn = Pattern.compile("[a-z0-9.!#$%&'*+/=?^_{|}~-]+@[a-z0-9-]+(\\.[a-z0-9-]+)*", Pattern.CASE_INSENSITIVE);
    var match = ptn.matcher(str);
    while (match.find()) {
      System.out.println(match.group());
    }
  }
}

MULTILINE

• Change the behavior of ^ and $
• ^ matches not only the beginning but also the number after the line break
• In case of $, it also matches the end of the line

import java.util.*;
import java.util.regex.Pattern;

public class Main {
  public static void main(String[] args) {
    var str = "Friends in the first grade\I wonder if n100 people can do it\n";
    // var ptn = Pattern.compile("^\\d*");
    var ptn = Pattern.compile("^\\d*", Pattern.MULTILINE);
    var match = ptn.matcher(str);
    while (match.find()) {
      System.out.println(match.group()); //1 100
    }
  }
}

DOTALL

• Change the behavior of .
• Matches all character strings including line breaks (\ n)
• Character strings can be matched as a single line (single line mode)

import java.util.*;
import java.util.regex.Pattern;

public class Main {
  public static void main(String[] args) {
    var str = "Wanted to meet\nWanted to meet\nWanted to meet\nYES";
    // var ptn = Pattern.compile("^.+");
    var ptn = Pattern.compile("^.+", Pattern.DOTALL);
    var match = ptn.matcher(str);
    while (match.find()) {
      System.out.println(match.group());
      //Wanted to meet
      //Wanted to meet
      //Wanted to meet
      //YES
    }
  }
}

Embedded flag

• The following code is the same
  • ?i ：CASE_INSENSITIVE
  • ?m ：MULTILINE
  • ?s ：DOTALL
  • ?u ：UNICODE_CASE
  • ?d ：UNIX_LINES
  • ?x ：COMMENTS

var ptn = Pattern.compile("(?i)[a-z0-9.!#$%&'*+/=?^_{|}~-]+@[a-z0-9-]+(\\.[a-z0-9-]+)*");

//var ptn = Pattern.compile("[a-z0-9.!#$%&'*+/=?^_{|}~-]+@[a-z0-9-]+(\\.[a-z0-9-]+)*", Pattern.CASE_INSENSITIVE);

Longest match / shortest match

• ** <. +> ** Longest match (match as long a string as possible)
• In <...>,. (Arbitrary character) is + (1 or more characters)
• All tag characters are output
• ** <. +?> ** Shortest match (match as short a string as possible)
• Output each tag character one by one

import java.util.regex.Pattern;

public class Main {
  public static void main(String[] args) {
    var tags = "<p><strong>NEKO</strong>site<a href='index.html'><img src='cat.jpg' /></a></p>";
    //Longest match
    //var ptn = Pattern.compile("<.+>"); //<p><strong>NEKO</strong>site<a href='index.html'><img src='cat.jpg' /></a></p>
    //Shortest match
    var ptn = Pattern.compile("<.+?>");
    var match = ptn.matcher(tags);
    while (match.find()) {
      System.out.println(match.group());
      //<p>
      //<strong>
      //</strong>
      //<a href='index.html'>
      //<img src='cat.jpg' />
      //</a>
      //</p>
    }
  }
}

Named capture

• Name and capture group
• Access with group ("name ")

import java.util.regex.Pattern;

public class Main {
  public static void main(String[] args) {
    var msg = "Mobile is 0123-99-It's 0000. Home is 000-123-4567 and so on.";
    var ptn = Pattern.compile("(?<area>\\d{2,4})-(?<city>\\d{2,4})-(?<local>\\d{4})");
    var match = ptn.matcher(msg);
    while (match.find()) {
      System.out.println("Starting position:" + match.start());
      System.out.println("End position:" + match.end());
      System.out.println("Matching string:" + match.group());
      System.out.println("Area code:" + match.group("area"));
      System.out.println("City code:" + match.group("city"));
      System.out.println("Subscriber number:" + match.group("local"));
      System.out.println("-----");
    }
  }
}

Back reference

• Matched strings can be referenced later with \\ 1

import java.util.regex.Pattern;
import java.util.regex.Matcher;
public class Main {
  //Back reference
  public static void main(String args[]){
    String str1 = "My name is <div>Neko</div>";
    String str2 = "I am a <span>Cat</span>";
    String str3 = "<span>Hello World</div>";
    String regex = "<(div|span)>.*?<\\/\\1>";
    Pattern p = Pattern.compile(regex);
    System.out.println("pattern: " + regex);
    check(p, str1);
    check(p, str2);
    check(p, str3);
  }
  private static void check(Pattern p, String target){
    Matcher m = p.matcher(target);

    if (m.find()){
      System.out.println("Match! " + target);
    }else{
      System.out.println("Unmatch! " + target);
    }
  }
}

Look-ahead / look-ahead

• Judge whether it matches by the presence or absence of the preceding and following character strings
• ** A (? = B) **: Matches A only if B immediately follows A
• ** A (?! B) **: Matches A only if B does not immediately follow A
• ** (? <= B) A **: Matches A only if there is B immediately before A
• ** (? <! B) A **: Matches A only if there is no B immediately before A

import java.util.regex.Pattern;
import java.util.regex.Matcher;
public class Main {

  private static void match(Pattern ptn, String input) {
    var match = ptn.matcher(input);
    while (match.find()) {
      System.out.println(match.group());
    }
    System.out.println("---");
  }

  public static void main(String[] args) {
    var re1 = Pattern.compile("My(?=Yes)"); 
    var re2 = Pattern.compile("My(?!Yes)"); 
    var re3 = Pattern.compile("(?<=。)My");  
    var re4 = Pattern.compile("(?<!。)My"); 
    var msg1 = "My yes is a cat";        
    var msg2 = "I am a cat. My name isn't there yet.";
    match(re1, msg1); //My
    match(re1, msg2); //---
    match(re2, msg1); //---
    match(re2, msg2); //My,My,My
    match(re3, msg1); //---
    match(re3, msg2); //My
    match(re4, msg1); //My
    match(re4, msg2); //My,My
  }
}

String replacement

• ** replaceAll method **
• The character string after replacement can include the character string before replacement
• $ 0 entire match string
• $ 1 First submatch string
• Named chapter groups available with $ {"name "}
• ** replaceFirst method **: Replace only the first string
• • ** replace method **: Replace fixed string

import java.util.regex.Pattern;
import java.util.regex.Matcher;
public class Main {

  public static void main(String[] args) {
    var str = "Click here for inquiries https://www.neko.com/is.";
    System.out.println(str.replaceAll(
        "(?i)http(s)?://([\\w-]+\\.)+[\\w-]+(/[\\\\w ./?%&=-]*)?",
        "<a href=\"$0\">$0</a>"));
     //Contact Us<a href="https://www.neko.com/">https://www.neko.com/</a>is.
  }
}

String split

• ** split method **
• cf: split of shear class compiles regular expression each time
• Use the Pattern class when reusing the same pattern

import java.util.regex.Pattern;
public class Main {
  //One or more digits+Split with
  public static void main(String[] args) {
    var str = "There are 2 chickens in the backyard and 22 chickens in the backyard.";
    var re = Pattern.compile("\\d{1,}Wow");
    var result = re.split(str);
    System.out.println(String.join(" ", result)); 
    //There is a chicken in the backyard
  }
}