Regular Expression in Java

Quick Study of Regex:

Common Syntax to be remember:

Regular Expression	Description
.	Matches any sign
^regex	regex must match at the beginning of the line
regex$	Finds regex must match at the end of the line
[abc]	Set definition, can match the letter a or b or c
[abc][vz]	Set definition, can match a or b or c followed by either v or z
[^abc]	When a "^" appears as the first character inside [] when it negates the pattern. This can match any character except a or b or c
[a-d1-7]	Ranges, letter between a and d and figures from 1 to 7, will not match d1
X|Z	Finds X or Z
XZ	Finds X directly followed by Z
$	Checks if a line end follows

Metacharacter:
e.g: use \d instead of [0-9]

Regular Expression	Description
\d	Any digit, short for [0-9]
\D	A non-digit, short for [^0-9]
\s	A whitespace character, short for [ \t\n\x0b\r\f]
\S	A non-whitespace character, for short for [^\s]
\w	A word character, short for [a-zA-Z_0-9]
\W	A non-word character [^\w]
\S+	Several non-whitespace characters

 Quantifiers:
 It defines how often an element can occurs.

Regular Expression	Description	Examples
*	Occurs zero or more times, is short for {0,}	X* - Finds no or several letter X, .* - any character sequence
+	Occurs one or more times, is short for {1,}	X+ - Finds one or several letter X
?	Occurs no or one times, ? is short for {0,1}	X? -Finds no or exactly one letter X
{X}	Occurs X number of times, {} describes the order of the preceding liberal	\d{3} - Three digits, .{10} - any character sequence of length 10
{X,Y}	Occurs between X and Y times,	\d{1,4}- \d must occur at least once and at a maximum of four
*?	? after a qualifier makes it a "reluctant quantifier", it tries to find the smallest match.

Grouping and Back reference:
Using (), one can group regular expressions. One can retrieve group values via $ i.e. one can refer to a group $1 is the first group, $2 the second, etc.
Lets for example assume you want to replace all whitespace between a letter followed by a point (dot) or a comma.

package com.Nur;

public class Testing {

    public static final String EXAMPLE_TEST = "This is my small example , full . nochange."
        + "string which I'm going to " + "use for pattern matching.";
   
   
    public static void main(String[] args) {
        String pattern = "(\\w)(\\s+)([\\.,])";
        System.out.println(EXAMPLE_TEST.replaceAll(pattern, "$3"));

    }

}

 Output: This is my small exampl, ful. nochange.string which I'm going to use for pattern matching.