How to use regular expressions in Java language

How to use regular expressions in Java language

Regular expression is a powerful text processing tool that can be used to match and verify text. In the Java language, regular expressions can also be used to achieve string matching and processing. This article will introduce how to use regular expressions in the Java language, covering the basic knowledge of regular expressions, commonly used regular expression syntax, and how to use regular expressions in Java programs.

1. Basic knowledge

1. What are regular expressions?

A regular expression is a text pattern used to describe a set of string matching rules. It consists of some ordinary characters and some special characters, and can be used to match strings that meet certain rules.

2. Advantages of regular expressions

The advantages of regular expressions include:

(1)Powerful text processing capabilities

Regular expressions can handle various complex patterns in strings, such as matching specific characters, numbers, or words.

(2) Write Simple

The syntax of regular expressions is relatively simple. Once you learn it, you can quickly write the required rules.

(3) Strong versatility

Regular expressions can not only be used in Java, but can also be applied to other programming languages ​​and text editors.

2. Commonly used regular expression syntax

The syntax of regular expressions is very complex. Here are some commonly used syntaxes.

1. Matching characters

is used to match any character, expressed as "."

2. Matching character set

is used to match any character in a set of characters, expressed as "[ ]".

For example: [abc] matches the characters "a" or "b" or "c".

3. Exclude character set

is used to match any character except the given character set, expressed as "¹".

For example: ² Matches any character except the characters "a", "b", and "c".

4. Match multiple characters

is used to match multiple repeated characters.

(1)* means matching 0 or more repeated characters.

For example: ab* matches "a", "ab" and "abb" etc.

(2) means matching one or more repeated characters.

For example: ab matches "ab" and "abb", etc., but does not match a single "a".

(3)? means matching 0 or 1 characters.

For example: ab? matches "a" or "ab".

5. Number of matching repetitions

is used to match the specified number of characters.

(1){n} means matching n repeated characters.

For example: a{3} matches "aaa".

(2){n,} means matching n or more repeated characters.

For example: a{3,} matches "aaa", "aaaa", etc.

(3){n,m} means matching n to m repeated characters.

For example: a{3,5} matches "aaa", "aaaa" or "aaaaa".

6. Boundary of matching string

Used to match the boundary of string.

(1)^ represents the starting position of the matching string.

For example: ^abc matches the string starting with the string "abc".

(2)$ represents the end position of the matching string.

For example: abc$ matches the string ending with the string "abc".

3. Using regular expressions in Java programs

In Java, you can use classes under the java.util.regex package to operate regular expressions. Commonly used classes and methods are introduced below.

1. Pattern class

The Pattern class represents the pattern in a regular expression. Pattern objects can be created using the Pattern.compile(String regex) method.

For example:

import java.util.regex.Pattern;
Pattern pattern = Pattern.compile("[abc]");

2. Matcher class

The Matcher class represents matching the specified string and pattern. Matcher objects can be created using the Pattern.matcher(String input) method.

For example:

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

Pattern pattern = Pattern.compile("[abc]");
Matcher matcher = pattern.matcher("abc");
while (matcher.find()) {
    System.out.println(matcher.group());
}

3. Regular expression method of String class

String class provides some methods to support regular expression processing, for example:

(1) matches(String regex): Determine whether the string matches the specified regular expression.

(2) replaceAll(String regex, String replacement): Replace the matching string with the specified replacement string.

For example:

String str = "The quick brown fox jumps over the lazy dog.";
String regex = "\s+";  // 匹配一个或多个空白字符
String replacement = ",";
String result = str.replaceAll(regex, replacement);
System.out.println(result);

The output result is:

The,quick,brown,fox,jumps,over,the,lazy,dog.

four , Summary

This article introduces how to use regular expressions in the Java language, including the basic knowledge and common syntax of regular expressions, as well as how to use regular expressions in Java programs. By learning regular expressions, we can handle string matching and processing more conveniently and improve programming efficiency.

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