An in-depth analysis of Java regular expressions: syntax of quantifiers, character classes, and boundary matching

Detailed explanation of Java regular expression syntax: quantifiers, character classes and boundary matching

Regular expressions are widely used in string matching and processing in Java. This article will introduce quantifiers, character classes and boundary matching in regular expressions in detail, and give specific code examples.

1. Quantifier

Quantifier is used to specify the number of repetitions of a matching rule. Commonly used quantifiers in Java include:

1. *: Matches the previous character 0 or more times.
2. : Matches the previous character 1 or more times.
3. ?: Match the previous character 0 or 1 times.
4. {n}: Match the previous character exactly n times.
5. {n,}: Match the previous character at least n times.
6. {n,m}: Match the previous character at least n times and at most m times.

The following is a specific example:

String str = "abcccdddeeeee";

String pattern1 = "c*";
System.out.println(str.matches(pattern1)); // 输出true

String pattern2 = "d+";
System.out.println(str.matches(pattern2)); // 输出true

String pattern3 = "e?";
System.out.println(str.matches(pattern3)); // 输出false

String pattern4 = "c{2}";
System.out.println(str.matches(pattern4)); // 输出false

String pattern5 = "c{3,}";
System.out.println(str.matches(pattern5)); // 输出true

String pattern6 = "c{2,4}";
System.out.println(str.matches(pattern6)); // 输出true

The str in the above code is a string containing different repeated characters. Next, some regular expression patterns are defined and matched using the matches() method. Depending on the quantifier, we can see different matching results.

2. Character class

The character class is used to specify a collection of characters. Character classes in Java are usually defined using []. You can use ranges, exclusions, and predefined characters to define a character class.

The following is a specific example:

String str = "abc123";

String pattern1 = "[a-c]";
System.out.println(str.matches(pattern1)); // 输出true

String pattern2 = "[0-9]";
System.out.println(str.matches(pattern2)); // 输出false

String pattern3 = "[a-c0-9]";
System.out.println(str.matches(pattern3)); // 输出true

String pattern4 = "[^a-z]";
System.out.println(str.matches(pattern4)); // 输出false

String pattern5 = "\d";
System.out.println(str.matches(pattern5)); // 输出false

String pattern6 = "\D";
System.out.println(str.matches(pattern6)); // 输出true

The str in the above code is a string containing letters and numbers. Next we define some regular expression patterns and use the matches() method to match. Depending on the character class, we can see different matching results.

3. Boundary matching

Boundary matching is used to limit a matching position. In Java, there are the following boundary matching symbols:

1. ^: Match string the starting position.
2. $: Matches the end position of the string.
3. : Match word boundaries.
4. B: Match non-word boundaries.

The following is a specific example:

String str = "hello world";

String pattern1 = "^hello";
System.out.println(str.matches(pattern1)); // 输出true

String pattern2 = "world$";
System.out.println(str.matches(pattern2)); // 输出true

String pattern3 = "\bhello";
System.out.println(str.matches(pattern3)); // 输出true

String pattern4 = "\Bworld";
System.out.println(str.matches(pattern4)); // 输出false

The str in the above code is a string containing a sentence. Next we define some regular expression patterns and use the matches() method to match. Depending on the boundary matching symbols, we can see different matching results.

Summary:

This article introduces quantifiers, character classes and boundary matching in Java regular expressions, and gives specific code examples. In actual programming, regular expressions are very useful tools that can simplify the matching and processing of strings. By mastering the basic usage of these regular expressions, you can use Java to process strings more efficiently.

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