Java Regex

Describe a text pattern, then choose the matching operation.

Step-by-step explanationsRunnable JavaPractice + answer

முக்கிய விளக்கங்கள் தமிழில் உள்ளன. Runnable example மற்றும் கூடுதல் practice notes ஆங்கிலத்தில் உள்ளன.

இந்தப் பாடத்தில்…

Explain the key idea behind Regular Expressions.
Read and run a complete example, then predict what changes.
Recognize the topic-specific mistake discussed below.

01 / The concept

அடிப்படையைப் புரிந்துகொள்ளுங்கள்

A regular expression describes a set of text patterns. matches checks the entire input; find searches for a matching region. Keep Java string escaping separate from regex escaping when a pattern contains backslashes.

02 / Step by step

விரிவான விளக்கங்கள்

Java Regular Expressions (Regex)

Regex என்பது characters-ஓட pattern — strings-ல search, replace, validate பண்றதுக்கு use ஆகுது.

Java-ல java.util.regex package use பண்றோம்:

Pattern — regex pattern compile பண்றதுக்கு
Matcher — pattern-ஐ string-ல match பண்றதுக்கு
PatternSyntaxException — invalid pattern-க்கு exception

Email validate பண்ற simple regex example:

Java reference fragmentJAVA

import java.util.regex.*;

// Basic find
Pattern p = Pattern.compile("codingtamilan", Pattern.CASE_INSENSITIVE);
Matcher m = p.matcher("I love CodingTamilan!");
System.out.println(m.find());   // true

// Full-string match
System.out.println("Hello".matches("[A-Z][a-z]+"));  // true

// replaceAll — remove non-digits
String cleaned = "Phone: 98-765-43210".replaceAll("\\D", "");
System.out.println(cleaned);   // 9876543210

Real-World Regex Examples

Use case	Pattern	Notes
Email	`[\w.-]+@[\w.-]+\.[a-zA-Z]{2,}`	Basic; use a library for production
Indian phone	`[6-9]\d{9}`	10-digit, starts 6–9
Digits only	`\d+`	One or more digits
Alphanumeric	`[a-zA-Z0-9]+`	Letters and digits
Date dd/mm/yyyy	`(0[1-9]\|[12]\d\|3[01])/(0[1-9]\|1[0-2])/\d{4}`	Strict day/month range

Java-வில் regex match செய்ய எந்த method பயன்படுத்துவோம்?

String.match()
String.matches() அல்லது Pattern.compile()
String.find()
String.regex()

Reveal answer / விடை

String.matches() அல்லது Pattern.compile()

`str.matches(regex)` whole string match செய்யும். Complex patterns-க்கு `Pattern.compile(regex)` மற்றும் `Matcher` class பயன்படுத்துவோம். Email validation, phone number validation-க்கு regex essential.

03 / Read and predict

முழுமையான உதாரணம்

This complete Main.java program focuses on the lesson idea. Reference fragments above may demonstrate individual statements; this example includes the imports, entry point, and supporting types it needs.

Main.java · complete runnable exampleJAVA

import java.util.regex.Pattern;
public class Main {
    public static void main(String[] args) throws Exception {
        System.out.println(Pattern.matches("[A-Z][0-9]{3}", "A123"));
        System.out.println(Pattern.matches("[A-Z][0-9]{3}", "A1234"));
    }
}

Predict first. Then reveal the expected output / வெளியீடு

true
false

The output shown was checked by running this complete example. Trace the executed statements in order, then compare with your run.

04 / Learn by changing

Change it. Run it. Understand it.

Load this lesson example into the editor. Change one value or statement, predict the result, then press RUN to execute Java.

Open Java editor ↗

After the editor loads, choose Load this lesson example, then press RUN.

Powered by OneCompiler. Code you run is sent to OneCompiler for execution. For Scanner examples, supply input in the editor I/O panel; for file examples, files exist in its execution environment.

05 / Your turn

நீங்களே முயற்சி செய்யுங்கள்

Practice challenge

Try a123 and B007. Which inputs satisfy the pattern?

Reveal the answer and reasoning / விடை

a123 fails because the first character is lowercase; B007 matches.

தவிர்க்க வேண்டிய தவறு: Pattern.matches is a whole-input check. It does not mean that any matching substring is enough.

பாடத்தின் சுருக்கம்

Explain the example without reading the output first.
Make the change requested in the practice task.
Describe why the result changed—or why compilation failed.

Ready to move on? Follow the next lesson in course order, or return to the curriculum to review a prerequisite.