Introduction to Regular Expressions

In Java, double backslashes (\\) are used to represent a single backslash (\) in regular expressions. Understanding greedy and non-greedy modes is crucial: - Greedy Mode: Matches the longest possible substring that fits the pattern.

• Non-Greedy Mode: Matches the shortest possible substring that fits the pattern.

Metacharacters

Quantifiers

• \\: Escapes special characters in regular expressions.
• *: Matches zero or more occurrences of the preceding element.
• +: Matches one or more occurrences of the preceding element.
• ?: Matches zero or one occurrence of the preceding element.
• {n}: Matches exactly n occurrences of the prceeding element.
• {n,}: Matches atleast n occurrences of the preceding element.
• {n,m}: Matches between n and m occurrences of the preceding element.
• (?i): Makes the match case-insensitive.

Alternation

• |: Acts as an OR operator between two patterns.

Grouping and Backreferences

• Capturing Groups: (...) for unnamed groups and (?<name>...)</name> or (?'name'...) for named groups.
• Non-Capturing Groups: (?:...).
• Positive Lookahead: (?=...).
• Negative Lookahead: (?!...).

Character Classes

• []: Defines a set of characters to match.
• [^]: Defines a set of characters not to match.
• -: Defines a range of characters within a character class.
• .: Matches any character except newline.
• \\d: Matches any digit (equivalent to [0-9]).
• \\D: Matches any non-digit (equivalent to [^0-9]).
• \\w: Matches any alphanumeric character and underscore (equivalent to [0-9a-zA-Z_]).
• \\W: Matches any non-alphanumeric character and underscore (equivalent to [^0-9a-zA-Z_]).
• \\s: Matches any whitespace character.
• \\S: Matches any non-whitespace character.

Anchors

• ^: Asserts position at start of a line.
• $: Asserts position at end of a line.
• \\b: Asserts position at a word boundary.
• \\B: Asserts position at a non-word boundary.

Greedy vs Non-Greedy Matching

Greedy Example in Java

String input = "<p class='active'>zhangSan</p>";
Pattern pattern = Pattern.compile("<p.*>");
Matcher matcher = pattern.matcher(input);
while (matcher.find()) {
    System.out.println(matcher.group(0)); // <p class='active'>zhangSan</p>
}

Non-Greedy Example in Java

String input = "<p class='active'>zhangSan</p>";
Pattern pattern = Pattern.compile("<p.*?>");
Matcher matcher = pattern.matcher(input);
while (matcher.find()) {
    System.out.println(matcher.group(0)); // <p class='active'>
}

Global Flags in JavaScript

• g: Global search.
• i: Case-insensitive search.

Global Flag Example in JavaScript

let text = 'from 2025-07-01 to 2025-07-03';
let regex = /\d{4}-\d{1,2}-\d{1,2}/g;
let match = regex.exec(text);
console.log(match[0]); // 2025-07-01
match = regex.exec(text);
console.log(match[0]); // 2025-07-03

Case-Insensitive Global Search Example in JavaScript

let text = 'Tom is tom';
let regex = /tom/ig;
let match = regex.exec(text);
console.log(match[0]); // Tom
match = regex.exec(text);
console.log(match[0]); // tom

Application: Date Parsing Tool in Java

public static Date convertToDate(String date) throws ParseException {
    String format = null;
    Locale locale = null;
    if (date.matches("^\\d{4}-\\d{2}-\\d{2}$")) {
        format = "yyyy-MM-dd";
    } else if (date.matches("^\\d{4}/\\d{2}/\\d{2}$")) {
        format = "yyyy/MM/dd";
    } else if (date.matches("^\\d{4}\\.\\d{2}\\.\\d{2}$")) {
        format = "yyyy.MM.dd";
    } else if (date.matches("^\\d{4}\\d{2}\\d{2}$")) {
        format = "yyyyMMdd";
    } else if (date.matches("^\\d{4}年\\d{2}月\\d{2}日$")) {
        format = "yyyy年MM月dd日";
    } else if (date.matches("^\\d{4}-\\d{1,2}-\\d{1,2} \\d{2}:\\d{2}:\\d{2}$")) {
        format = "yyyy-MM-dd HH:mm:ss";
    } else if (date.matches("^[A-Z][a-z]{2} [A-Z][a-z]{2} \\d{2} \\d{2}:\\d{2}:\\d{2} CST \\d{4}$")) {
        format = "EEE MMM dd HH:mm:ss 'CST' yyyy";
        locale = Locale.US;
    } else if (date.matches("^[A-Z][a-z]{2} [A-Z][a-z]{2} \\d{2} \\d{2}:\\d{2}:\\d{2} GMT\\+08:00 \\d{4}$")) {
        format = "EEE MMM dd HH:mm:ss z yyyy";
        locale = Locale.ENGLISH;
    }
    SimpleDateFormat sdf = (locale == null) ? new SimpleDateFormat(format) : new SimpleDateFormat(format, locale);
    return sdf.parse(date);
}