# Java Regex Concepts in Java

**Introduction**

**Regex (Regular Expression)** is a sequence of characters used to search, match, validate, or manipulate text based on a specific pattern.

Java provides the `Pattern` and `Matcher` classes in the `java.util.regex` package.

* * *

**1\. Regex Syntax**

Basic syntax:

```java
Pattern pattern = Pattern.compile("regex");
Matcher matcher = pattern.matcher("text");
```

Example:

```java
Pattern pattern = Pattern.compile("Java");
Matcher matcher = pattern.matcher("I am learning Java");

System.out.println(matcher.find());
```

Output:

```text
true
```

* * *

**2\. Common Regex Patterns**

| Regex | Meaning |
| --- | --- |
| `.` | Any character |
| `\\d` | Digit |
| `\\D` | Non-digit |
| `\\w` | Word character |
| `\\s` | Whitespace |
| `+` | One or more |
| `*` | Zero or more |
| `?` | Zero or one |
| `^` | Start of string |
| `$` | End of string |

* * *

**3\. Check Digits**

```java
String input = "12345";

System.out.println(input.matches("\\d+"));
```

Output:

```text
true
```

* * *

**4\. Check Alphabetic Characters**

```java
String input = "Java";

System.out.println(input.matches("[a-zA-Z]+"));
```

Output:

```text
true
```

* * *

**5\. Validate Mobile Number**

Example for a 10-digit number:

```java
String mobile = "9876543210";

System.out.println(mobile.matches("\\d{10}"));
```

Output:

```text
true
```

* * *

**6\. Validate Email**

```java
String email = "user@gmail.com";

String regex = "^[A-Za-z0-9+_.-]+@[A-Za-z0-9.-]+$";

System.out.println(email.matches(regex));
```

Output:

```text
true
```

* * *

**7\. Using Pattern and Matcher**

```java
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class RegexExample {
    public static void main(String[] args) {

        String text = "Java is powerful";

        Pattern pattern = Pattern.compile("Java");
        Matcher matcher = pattern.matcher(text);

        if (matcher.find()) {
            System.out.println("Pattern found");
        }
    }
}
```

Output:

```text
Pattern found
```

* * *

**8\. Useful Regex Methods**

### `matches()`

Checks whether the complete string matches the pattern.

```java
System.out.println("12345".matches("\\d+"));
```

### `find()`

Searches for the pattern inside a string.

```java
Matcher matcher = Pattern.compile("Java")
        .matcher("I am learning Java");

System.out.println(matcher.find());
```

### `replaceAll()`

Replaces matching characters.

```java
String result = "Java123".replaceAll("\\d", "");

System.out.println(result);
```

Output:

```text
Java
```

* * *

**9\. Common Regex Examples**

```text
\\d+              → One or more digits
[a-zA-Z]+         → Only alphabets
\\d{10}            → 10 digits
^[0-9]+$           → Only numbers
^[A-Z].*           → Starts with uppercase letter
\\s+               → One or more spaces
```

* * *

**Conclusion**

Java Regex is useful for **pattern matching, text searching, input validation, and text replacement**.

The main classes are:

*   `Pattern` → Defines the regex pattern
    
*   `Matcher` → Finds and matches the pattern
    
*   `matches()` → Checks the complete string
    
*   `find()` → Searches for a pattern
    
*   `replaceAll()` → Replaces matching text