What are the java regular expression syntax

The basic syntax of Java regular expressions includes metacharacters, predefined patterns, character classes, quantifier modifiers, escaping, grouping, non-capturing grouping, look-ahead assertions, named capture groups, backward references, etc. Detailed introduction: 1. Metacharacters: ".": matches any single character (except newline); "^": matches the beginning of the input string; "$": matches the end of the input string; "*": Matches the previous subexpression zero or more times; " ": matches the previous subexpression one or more times; "?": matches the previous subexpression zero or one time, etc.

Operating system for this tutorial: Windows 10 system, Dell G3 computer.

Java Regular Expression (Regular Expression) is a powerful text processing tool that uses specific patterns to match, find, or manipulate text. Here is some basic syntax for Java regular expressions:

1. Metacharacters:

   • .: Matches any single character (except newline).
   • ^: Matches the beginning of the input string.
   • $: Matches the end position of the input string.
   • *: Matches the previous subexpression zero or more times.
   • : Matches the previous subexpression one or more times.
   • ?: Matches the previous subexpression zero or one time.
   • {n}: n is a non-negative integer. Match determined n times.
   • {n,}: n is a non-negative integer. Match at least n times.
   • {n,m}: m and n are both non-negative integers, where n
   • \d: Matches a numeric character. Equivalent to [0-9].
   • \D: Matches a non-numeric character. Equivalent to [^0-9].
   • \s: Matches any whitespace characters, including spaces, tabs, form feeds, etc. Equivalent to [\f\n\r\t\v].
   • \S: Matches any non-whitespace character. Equivalent to [^\f\n\r\t\v].
   • \w: Matches any letter, number, or underline character including an underscore. Equivalent to '[A-Za-z0-9_]'.
   • \W: Matches any non-word character. Equivalent to '[^A-Za-z0-9_]'.

2. Predefined patterns:

   • ^$: Empty line (consisting only of whitespace characters).
   • .: Any single character (except newline).
   • \n: newline character.
   • \r: carriage return character.
   • \t: Tab character.

3. Character class:

   • []: Character set, such as [abc] will match any character in a, b or c.
   • [^]: Negative character set, such as [^abc] will match any character except a, b, c.

4. Modifiers of quantifiers:

   • ?: The previous character or subexpression appears 0 or 1 times.
   • *: The previous character or subexpression appears 0 or more times.
   • : The previous character or subexpression appears 1 or more times.
   • {n}: n is a non-negative integer, and the previous character or subexpression appears exactly n times.
   • {n,}: n is a non-negative integer, and the previous character or subexpression appears at least n times.
   • {n,m}: m and n are both non-negative integers, where n

5. Escape: Use backslash (\) to escape special characters, such as \. to match actual dot characters instead of special characters.

6. Grouping: Use parentheses for grouping, for example (ab)* means matching "ab" zero or more times.

7. Non-capturing grouping: Use parentheses and add a question mark in front to implement non-capturing grouping, for example (?:ab)* means matching "ab" zero or more times, But the group contents are not captured.

8. Look-ahead assertion: Use (?=...) and (?!...) to make a look-ahead assertion, for example (?=abc) represents a string that must follow immediately It is "abc", and (?!abc) means that the string immediately following cannot be "abc".

9. Named capturing group: Use (?...) to define a named capturing group, for example (?202[0-9]{2}) Represents matching years between 2020 and 2029 and capturing them as a group named "year".

10. Back reference: Use \n (where n is a positive integer) for back reference, referencing the matching content of the nth capture group, such as continuous occurrences in the pattern string The same character twice can be represented by a back reference. For example, the same character that appears twice in a row in a pattern string can be represented by a back reference. For example, the same character that appears twice in a pattern string can be represented by a back reference. Quote

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