The basic regular expression API in Java

The basic regular expression in Java involved 2 classes, java.util.regex.Pattern and  java.util.regex.Matcher. The most basic usage has 3 steps.

• Create a pattern instance
• Create a matcher instance from pattern instance. The String  under test is set in this step
• Check match result using methods from matcher

The following demo code is a hello world level example on how to use Java regular express API.

package com.shengwang.demo;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class HelloRegex {

  public static void main(String[] args) {
    String input = "Hello World 123#456";
    // -------------------------------------------
    // Step 1, define Pattern instance
    // -------------------------------------------
    Pattern p = Pattern.compile("(\\d+)"); // use static method

    // -------------------------------------------
    // Step 2, define matcher instance
    // -------------------------------------------
    Matcher m = p.matcher(input); // create matcher from pattern

    // -------------------------------------------
    // Step3, use loop to go through every match
    // -------------------------------------------
    while (m.find()) {
      System.out.println("" + m.group(1));
    }
  }
}

In this demo, we use method find()  to do the real match, there are 2 other method matches() and lookingAt(). All these 3 methods return boolean indicates match success or fail. The differences among them are:

•  matches()  try to match the whole input. 
• find() try to match a substring of the input.
  
• lookingAt() try to match a substring of the input must at beginning of the input.(Think it as a startWith operation)

When using  Pattern + Matcher class, normally means we want to do some manipulation on the matched result. If you only want to get a boolean result to verify a input String, there is no need to use class Pattern + Matcher, use method matches() from String class instead.

  String input = "888ABC999";
  boolean  matchResult = input.matches("\\d+ABC\\d+"); // match whole string