Regular Expressions (Regex)

1. Definition

Regular Expression (Regex) is a pattern used to search, match, validate, or extract text from a string.

In Java and Spring applications, regex is commonly used for:

• Input validation
• Data parsing
• Log processing
• Text extraction
• Request validation
• Data cleaning

Example:

String text = "Order123";
text.matches("\\d+");  // false

2. Where Regex is Used in Java / Spring

Area	Example
Input validation	Email, phone number
Spring validation	@Pattern annotation
Parsing logs	Extract IDs or timestamps
Data processing	Extract numbers from text
Replace operations	Remove unwanted characters
Security	Password validation
API request validation	Validate request payload fields

3. Java Regex Classes

Java provides classes in java.util.regex package.

Class	Purpose
Pattern	Compiled regex pattern
Matcher	Performs matching
Pattern.compile()	Compile regex
matcher()	Apply regex on text
find()	Find next match
matches()	Check full string match
group()	Get matched text

4. Character Classes

Match one character from a set.

Regex	Meaning	Example
[abc]	a OR b OR c	a
[a-z]	lowercase letters	hello
[A-Z]	uppercase letters	JAVA
[0-9]	digits	123
[a-zA-Z]	letters	Java

Example

String text = "abc123";

Pattern p = Pattern.compile("[a-z]");
Matcher m = p.matcher(text);

while(m.find()){
    System.out.println(m.group());
}

Output

a
b
c

---

5. Predefined Character Classes

Shortcuts for common patterns.

Regex	Meaning	Equivalent
\d	digit	[0-9]
\D	non digit	[^0-9]
\w	word character	[a-zA-Z0-9_]
\W	non word char	special chars
\s	whitespace	space/tab
\S	non whitespace	letters

---

6. Quantifiers (Repetition)

Control how many times a pattern appears.

Regex	Meaning	Example
*	0 or more	a*
+	1 or more	a+
?	0 or 1	a?
{n}	exactly n	\d{4}
{n,}	at least n	\d{2,}
{n,m}	between n and m	\d{2,5}

String pin = "1234";
boolean valid = pin.matches("\\d{4}");  //true

7. Anchors

Define position in string.

Regex	Meaning
^	start of string
$	end of string
\b	word boundary

“12345”.matches(“^\d+$”); // true

Matches only digits.

8. Grouping

Groups allow capturing values.

Pattern p = Pattern.compile("(\\d{4})-(\\d{2})-(\\d{2})");
Matcher m = p.matcher("2026-03-05");

if(m.find()){
    System.out.println(m.group(1)); // year
    System.out.println(m.group(2)); // month
    System.out.println(m.group(3)); // day
}

9. Java String Regex Methods

Method	Purpose
matches()	full string match
replaceAll()	replace regex match
split()	split using regex

matches()

“1234”.matches(“\d+”); //true

replaceAll()

String text = "abc123";
text.replaceAll("\\d", "");

Result : abc

split()

String data = "one,two,three";
String[] arr = data.split(",");

10. Regex with Java Streams

    List<String> list = List.of("abc","123","xyz","456","-120");
    
    List<String> result = list.stream()
        .filter(s -> s.matches("-?\\d+"))
        .toList();
    System.out.println(result); //[123, 456, -120]

Regex Testing Tools

Useful tools

• https://regex101.com
• https://regexr.com