Subexpression	Matches
^	Matches beginning of line.
$	Matches end of line.
.	Matches any single character except newline. Using m option allows it to match newline as well.
[...]	Matches any single character in brackets.
[^...]	Matches any single character not in brackets
\A	Beginning of entire string
\z	End of entire string
\Z	End of entire string except allowable final line terminator.
re*	Matches 0 or more occurrences of preceding expression.
re+	Matches 1 or more of the previous thing
re?	Matches 0 or 1 occurrence of preceding expression.
re{ n}	Matches exactly n number of occurrences of preceding expression.
re{ n,}	Matches n or more occurrences of preceding expression.
re{ n, m}	Matches at least n and at most m occurrences of preceding expression.
a| b	Matches either a or b.
(re)	Groups regular expressions and remembers matched text.
(?: re)	Groups regular expressions without remembering matched text.
(?> re)	Matches independent pattern without backtracking.
\w	Matches word characters.
\W	Matches nonword characters.
\s	Matches whitespace. Equivalent to [\t\n\r\f].
\S	Matches nonwhitespace.
\d	Matches digits. Equivalent to [0-9].
\D	Matches nondigits.
\A	Matches beginning of string.
\Z	Matches end of string. If a newline exists, it matches just before newline.
\z	Matches end of string.
\G	Matches point where last match finished.
\n	Back-reference to capture group number "n"
\b	Matches word boundaries when outside brackets. Matches backspace (0x08) when inside brackets.
\B	Matches nonword boundaries.
\n, \t, etc.	Matches newlines, carriage returns, tabs, etc.
\Q	Escape (quote) all characters up to \E
\E	Ends quoting begun with \Q

SN	Methods with Description
1	public int start() Returns the start index of the previous match.
2	public int start(int group) Returns the start index of the subsequence captured by the given group during the previous match operation.
3	public int end() Returns the offset after the last character matched.
4	public int end(int group) Returns the offset after the last character of the subsequence captured by the given group during the previous match operation.

SN	Methods with Description
1	public boolean lookingAt() Attempts to match the input sequence, starting at the beginning of the region, against the pattern.
2	public boolean find() Attempts to find the next subsequence of the input sequence that matches the pattern.
3	public boolean find(int start Resets this matcher and then attempts to find the next subsequence of the input sequence that matches the pattern, starting at the specified index.
4	public boolean matches() Attempts to match the entire region against the pattern.

SN	Methods with Description
1	public Matcher appendReplacement(StringBuffer sb, String replacement) Implements a non-terminal append-and-replace step.
2	public StringBuffer appendTail(StringBuffer sb) Implements a terminal append-and-replace step.
3	public String replaceAll(String replacement) Replaces every subsequence of the input sequence that matches the pattern with the given replacement string.
4	public String replaceFirst(String replacement) Replaces the first subsequence of the input sequence that matches the pattern with the given replacement string.
5	public static String quoteReplacement(String s) Returns a literal replacement String for the specified String. This method produces a String that will work as a literal replacement s in the appendReplacement method of the Matcher class.

SN	Methods with Description
1	public String getDescription() Retrieves the description of the error.
2	public int getIndex() Retrieves the error index.
3	public String getPattern() Retrieves the erroneous regular expression pattern.
4	public String getMessage() Returns a multi-line string containing the description of the syntax error and its index, the erroneous regular expression pattern, and a visual indication of the error index within the pattern.

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

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

      // String to be scanned to find the pattern.
      String line = "This order was placed for QT3000! OK?";
      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) );
         System.out.println("Found value: " + m.group(1) );
         System.out.println("Found value: " + m.group(2) );
      } else {
         System.out.println("NO MATCH");
      }
   }
}

						
The 

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

public class RegexMatches
{
    private static final String REGEX = "\\bcat\\b";
    private static final String INPUT =
                                    "cat cat cat cattie cat";

    public static void main( String args[] ){
       Pattern p = Pattern.compile(REGEX);
       Matcher m = p.matcher(INPUT); // get a matcher object
       int count = 0;

       while(m.find()) {
         count++;
         System.out.println("Match number "+count);
         System.out.println("start(): "+m.start());
         System.out.println("end(): "+m.end());
      }
   }
}

						
The 

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

public class RegexMatches
{
    private static final String REGEX = "foo";
    private static final String INPUT = "fooooooooooooooooo";
    private static Pattern pattern;
    private static Matcher matcher;

    public static void main( String args[] ){
       pattern = Pattern.compile(REGEX);
       matcher = pattern.matcher(INPUT);

       System.out.println("Current REGEX is: "+REGEX);
       System.out.println("Current INPUT is: "+INPUT);

       System.out.println("lookingAt(): "+matcher.lookingAt());
       System.out.println("matches(): "+matcher.matches());
   }
}

						

Current REGEX is: foo
Current INPUT is: fooooooooooooooooo
lookingAt(): true
matches(): false

						
The 

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

public class RegexMatches
{
    private static String REGEX = "dog";
    private static String INPUT = "The dog says meow. " +
                                    "All dogs say meow.";
    private static String REPLACE = "cat";

    public static void main(String[] args) {
       Pattern p = Pattern.compile(REGEX);
       // get a matcher object
       Matcher m = p.matcher(INPUT); 
       INPUT = m.replaceAll(REPLACE);
       System.out.println(INPUT);
   }
}

						

The cat says meow. All cats say meow.

						
The 

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

public class RegexMatches
{
   private static String REGEX = "a*b";
   private static String INPUT = "aabfooaabfooabfoob";
   private static String REPLACE = "-";
   public static void main(String[] args) {
      Pattern p = Pattern.compile(REGEX);
      // get a matcher object
      Matcher m = p.matcher(INPUT);
      StringBuffer sb = new StringBuffer();
      while(m.find()){
         m.appendReplacement(sb,REPLACE);
      }
      m.appendTail(sb);
      System.out.println(sb.toString());
   }
}

						

-foo-foo-foo-