Commonly used regular expression syntax and usage tips in Java

Common grammar and usage skills of Java regular expressions

Introduction:
Regular Expression (Regular Expression) is a powerful text matching tool that can easily Used for string search, replacement and validation. Regular expressions in Java are supported by the java.util.regex package. This article will introduce the common syntax and usage techniques of Java regular expressions, and provide specific code examples.

1. Common syntax
   1.1 Character class
   Character Classes (Character Classes) represent matching a specified group of characters.
2. Use [] to represent character classes, such as [abc] to match any one of the characters a, b, and c.
3. Use [^] to negate, for example ¹ means to match any character except characters a, b, c.
4. Use - to indicate a range, such as [a-z] to match any lowercase letter from a to z.

1.2 Predefined Character Classes
Predefined Character Classes are the abbreviations of some commonly used character classes.

• d means matching any number, equivalent to [0-9].
• D means matching any character except numbers, which is equivalent to ².
• w means to match any letter, number or underscore, equivalent to [a-zA-Z_0-9].
• W means to match any character except letters, numbers and underscores, which is equivalent to ³.
• s means matching any whitespace character, including spaces, tabs, newlines, etc.
• S ​​means matching any character except whitespace characters.

1.3 Qualifier
Quantifiers (Quantifiers) are used to specify the number of matching characters.

• • means matching 0 or more characters.
• • # means matching 1 or more characters.
• ? means matching 0 or 1 characters.
• {n} means matching exactly n characters, for example, d{3} means matching exactly 3 numbers.
• {n,} means matching at least n characters.
• {n,m} means matching n to m characters.

1.4 Escape Characters
Escape Characters are used for special characters in regular expressions.

• Escape the following characters, such as . to match dot characters.
• \ means match slash characters.
• means matching tab characters.
• means matching newline characters.
• means matching the carriage return character.

2. Usage Tips
   2.1 Find matches
   Use the find method of the Matcher class to find matches.

   Pattern pattern = Pattern.compile("a.b");
   Matcher matcher = pattern.matcher("acb");
   if (matcher.find()) {
    System.out.println("找到匹配");
   } else {
    System.out.println("未找到匹配");
   }

2.2 Extract matches
Use the group method of the Matcher class to extract matches.

Pattern pattern = Pattern.compile("\d+");
Matcher matcher = pattern.matcher("abc123def");
if (matcher.find()) {
    String match = matcher.group();
    System.out.println("提取到的匹配项为：" + match);
}

2.3 Replacement matching
Use the replaceAll method of the String class to replace matching items.

String str = "abc123def";
str = str.replaceAll("\d+", "数字");
System.out.println("替换后的字符串为：" + str);

2.4 Verify input
Use the matches method of the String class to verify whether the input matches the specified regular expression.

String input = "abc123";
if (input.matches("[a-z]+\d+")) {
    System.out.println("输入合法");
} else {
    System.out.println("输入非法");
}

Conclusion:
This article introduces the common syntax and usage techniques of Java regular expressions, and provides specific code examples. By learning and mastering the basic syntax and usage skills of regular expressions, you can search, replace and verify strings more efficiently. I hope this article is helpful to your study.

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1. abc ↩
2. 0-9 ↩
3. a-zA-Z_0-9 ↩

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