Regular Expression

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To use java regular expression you need to be familiar with java.util.regex.Pattern and java.util.regex.Matcher classes. Here is a short code on how to check if an IP address is valid.

	// IPv4 Regular Expression Pattern
	String IPV4PATTERN = "^(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})$";
	
	// Compile the string pattern
	Pattern pattern = Pattern.compile(IPV4PATTERN);
		
	// IPs to check
	String[] ips = new String[] { "192.168.1.1", "192.168.1.1000", "itcuties.com"};
		
		
	// Check IPs
	for (String ip: ips) {
		// Create the matcher for a given IP
		Matcher matcher = pattern.matcher(ip);
		System.out.println(ip + " is " + (matcher.matches() ? "valid":"invalid"));
	}

This code gives the following output:

	192.168.1.1 is valid
	192.168.1.1000 is invalid
	itcuties.com is invalid

At the beginning a regular expression pattern needs to be compiled by calling java.util.regex.Pattern.compile method which takes a regular expression string pattern as argument. Next we need a java.util.regex.Matcher object which is an engine that performs match operations on character sequences by interpreting the given pattern. By calling java.util.regex.Matcher.matches method a given string is validated with the pattern.

Here are some useful java regular expression patterns:
- IPv4 regular expression pattern
- URL regular expression pattern
- EMAIL regular expression pattern

IPv4 regular expression pattern

"^(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})$"

URL regular expression pattern

"^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\\?([^#]*))?(#(.*))?"

EMAIL regular expression pattern

"^[_A-Za-z0-9-\\+]+(\\.[_A-Za-z0-9-]+)*@[A-Za-z0-9-]+(\\.[A-Za-z0-9]+)*(\\.[A-Za-z]{2,})$"