Module 2: Intro to RegEx

Learning Objectives

• Reading data from files into strings
• Parsing strings into classes using RegEx
• RegEx for digits
• RegEx for words
• RegEx capture groups
• RegEx named capture groups
• RegEx for whitespace
• RegEx quantifiers

RegEx Fundamentals

Regular expressions (RegEx) are sequences of characters that define a search pattern. They are used for pattern matching within text and are supported in most programming languages, including Java.

Common RegEx Patterns

Here are some common patterns used in regular expressions:

• \d - Matches any digit (0-9)
• \w - Matches any word character (alphanumeric plus underscore)
• \s - Matches any whitespace character (spaces, tabs, line breaks)
• . - Matches any character except newline
• * - Matches 0 or more of the preceding element
• + - Matches 1 or more of the preceding element
• ? - Matches 0 or 1 of the preceding element
• {n} - Matches exactly n occurrences of the preceding element
• {n,} - Matches n or more occurrences of the preceding element
• {n,m} - Matches between n and m occurrences of the preceding element

Capture Groups

Capture groups allow you to extract parts of the matched text:

• (pattern) - Creates a capture group with the matched pattern
• (?<name>pattern) - Creates a named capture group

Example Implementation

Here's how you might use RegEx in Java for various common tasks:

// Simple pattern matching
String text = "Java Regular Expressions";
boolean matches = text.matches(".*Regular.*"); // true

// Finding all matches in a string
String text = "Contact us at support@example.com or sales@example.com";
Pattern pattern = Pattern.compile("[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}");
Matcher matcher = pattern.matcher(text);

while (matcher.find()) {
    System.out.println("Found email: " + matcher.group());
}

// Using capture groups to extract data
String dateString = "Today is 2023-05-15";
Pattern pattern = Pattern.compile("(\\d{4})-(\\d{2})-(\\d{2})");
Matcher matcher = pattern.matcher(dateString);

if (matcher.find()) {
    String year = matcher.group(1);  // 2023
    String month = matcher.group(2); // 05
    String day = matcher.group(3);   // 15
    System.out.println("Year: " + year + ", Month: " + month + ", Day: " + day);
}

// Using named capture groups
String nameString = "John Doe";
Pattern pattern = Pattern.compile("(?<firstName>\\w+)\\s(?<lastName>\\w+)");
Matcher matcher = pattern.matcher(nameString);

if (matcher.find()) {
    String firstName = matcher.group("firstName"); // John
    String lastName = matcher.group("lastName");   // Doe
    System.out.println("First name: " + firstName + ", Last name: " + lastName);
}