abc…	Letters
123…	Digits
\d	Any Digit
\D	Any Non-digit character
.	Any Character
\.	Period
[abc]	Only a, b, or c
[^abc]	Not a, b, nor c
[a-z]	Characters a to z
[0-9]	Numbers 0 to 9
\w	Any Alphanumeric character
\W	Any Non-alphanumeric character
{m}	m Repetitions
{m,n}	m to n Repetitions

-	Zero or more repetitions
+	One or more repetitions
?	Optional character
\s	Any Whitespace
\S	Any Non-whitespace character
^…$	Starts and ends
(…)	Capture Group
(a(bc))	Capture Sub-group
(.*)	Capture all
(abc|def)	Matches abc or def

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

/**
 * By this example you should know how to find a digit string fro the given alphanumeric string 
 **/

public class RegexMatches{
	public static void main(String args[]){
		// String to be scanned to find the pattern.
		String line ="This order was places for QT3000! OK?";
		/**
		 *  . matches any single character except newline
		 *  re* mathches 1 ore more occurrences of preceding expression
		 *  re+ matches 1 ore more of the previous thing
		 *  \d matches digits
		 */
		String pattern ="(.*)(\\d+)(.*)";   
		// Create a Pattern object
		Pattern r =Pattern.compile(pattern);
		// Now create matcher object.
		Matcher m = r.matcher(line);
		if(m.find()){
			System.out.println("Found value: "+ m.group(0));  // () means a group
			System.out.println("Found value: "+ m.group(1));
			System.out.println("Found value: "+ m.group(2));
		}else{
			System.out.println("NO MATCH");
		}
	}	
}