Java Regex Syntax Rules Explained

The syntax rules of Java regular expressions are as follows:

1. Character class: Use square brackets to specify a character class, which can match any of the characters within the brackets. For example, [abc] can match the characters ‘a’, ‘b’, or ‘c’.
2. Range: Use a hyphen (-) to specify a range, which can match any character within the range on either side of the hyphen. For example, [0-9] can match any numeric character.
3. Negated character class: Using the caret (^) in the first position within square brackets specifies a negated character class, which can match any character except those listed within the brackets. For example, [^abc] can match any character except ‘a’, ‘b’, or ‘c’.
4. Predefined character classes simplify matching common characters in Java regular expressions. For example, \d matches any digit, \w matches any letter, digit, or underscore, and \s matches any whitespace character.
5. Boundary matcher: used to match the boundaries of a string. For example, ^ can match the beginning of a string, $ can match the end of a string.
6. Quantifiers: used to specify the number of matches. For example, * means matching zero or more times, + means matching one or more times, ? means matching zero or one time, {n} means matching exactly n times, {n,} means matching at least n times, and {n,m} means matching at least n times but not exceeding m times.
7. Escape characters are used to escape characters that have special meanings. For example, “.” can match a period character (.), “\” can match a backslash character (\).
8. Grouping: Using parentheses to group allows multiple characters to be combined together for matching or operations. For example, (abc|def) can match the character sequence ‘abc’ or ‘def’.
9. Backward referencing: used to refer to groups that have already been matched earlier. For example, \1 represents the first group, \2 represents the second group.
10. Zero-width assertion: used to match a position without consuming characters. For example, (?=pattern) matches the position immediately following pattern, while (?!pattern) matches a position not immediately following pattern.

These are some common syntax rules for Java regular expressions, with advanced features and usage that need to be learned and applied based on specific needs.