In Java, by default, the regular expression (regex) matching is case sensitive. To enable the regex case insensitive matching, add (?) prefix or enable the case insensitive flag directly in the Pattern.compile().

A regex pattern example.

Pattern = Registrar:\\s(.*)

Case Insensitive, add (?) prefix.

Pattern = (?)Registrar:\\s(.*)

Case Insensitive, add Pattern.CASE_INSENSITIVE flag.

Pattern.compile("Registrar:\\s(.*)", Pattern.CASE_INSENSITIVE);

1. RegEx Example

Example to use regex case insensitive matching to get the “Registrar” information.

package com.mkyong.regex
import java.util.ArrayList;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class RunExampleTest{
	private Pattern registrarPattern = Pattern.compile(REGISTRAR_PATTERN);
	//alternative
	/*private Pattern registrarPattern = 
		Pattern.compile(REGISTRAR_PATTERN, Pattern.CASE_INSENSITIVE);*/
	private Matcher matcher;
	private static final String REGISTRAR_PATTERN = "(?)Registrar:\\s(.*)";
	public static void main(String[] args) {
		String data = "Testing... \n" +
			"Registrar: abc.whois.com\n" +
			"registrar: 123.whois.com\n" +
			"end testing";
		RunExampleTest obj = new RunExampleTest();
		List<String> list = obj.getRegistrar(data);
		System.out.println(list);
	private List<String> getRegistrar(String data){
		List<String> result = new ArrayList<String>();
		matcher = registrarPattern.matcher(data);
		while (matcher.find()) {
			result.add(matcher.group(1));
		return result;

Output

[abc.whois.com, 123.whois.com]

References

1. Wikipedia : Case Sensitivity
2. Wikipedia : Regular Expression/