added regex

Author: connectaman

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README.md

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+### Java Regex
+
+The Java Regex or Regular Expression is an API to define a pattern for searching or manipulating strings.
+
+It is widely used to define the constraint on strings such as password and email validation. After learning Java regex tutorial, you will be able to test your regular expressions by the Java Regex Tester Tool.
+
+Java Regex API provides 1 interface and 3 classes in java.util.regex package.
+
+##### java.util.regex package
+
+The Matcher and Pattern classes provide the facility of Java regular expression. The java.util.regex package provides following classes and interfaces for regular expressions.
+
+   1- MatchResult interface
+   
+   2- Matcher class
+   
+   3- Pattern class
+   
+   4- PatternSyntaxException class
+   
+![](https://static.javatpoint.com/images/java-regex-api.jpg)
+
+
+
+------
+
+##### Matcher class
+
+It implements the MatchResult interface. It is a regex engine which is used to perform match operations on a character sequence.
+
+|No.|	Method|	Description|
+|-----|-----|--------|
+|1|	boolean matches()|	test whether the regular expression matches the pattern.|
+|2|	boolean find()|	finds the next expression that matches the pattern.|
+|3|	boolean find(int start)|	finds the next expression that matches the pattern from the given start number.|
+|4|	String group()	|returns the matched subsequence.|
+|5|	int start()	|returns the starting index of the matched subsequence.|
+|6|	int end()	|returns the ending index of the matched subsequence.|
+|7|	int groupCount()	|returns the total number of the matched subsequence.|
+
+-------
+
+##### Pattern class
+
+It is the compiled version of a regular expression. It is used to define a pattern for the regex engine.
+
+|No.|	Method|	Description|
+|-----|------|-------|
+|1	|static Pattern compile(String regex)	|compiles the given regex and returns the instance of the Pattern.|
+|2	|Matcher matcher(CharSequence input)|	creates a matcher that matches the given input with the pattern.|
+|3	|static boolean matches(String regex, CharSequence input)|	It works as the combination of compile and matcher methods. It compiles the regular expression and matches the given input with the pattern.|
+|4	|String[] split(CharSequence input)	|splits the given input string around matches of given pattern.|
+|5	|String pattern()	|returns the regex pattern.|
+
+------
+
+##### Regular Expression . Example
+
+```java
+    import java.util.regex.*;  
+    class RegexExample2{  
+    public static void main(String args[]){  
+    System.out.println(Pattern.matches(".s", "as"));//true (2nd char is s)  
+    System.out.println(Pattern.matches(".s", "mk"));//false (2nd char is not s)  
+    System.out.println(Pattern.matches(".s", "mst"));//false (has more than 2 char)  
+    System.out.println(Pattern.matches(".s", "amms"));//false (has more than 2 char)  
+    System.out.println(Pattern.matches("..s", "mas"));//true (3rd char is s)  
+    }
+}  
+```
+
+##### Regex Character classes
+
+|No.	|Character Class|	Description|
+|-----|-------|---------|
+|1|	[abc]	|a, b, or c (simple class)|
+|2|	[^abc]	|Any character except a, b, or c (negation)|
+|3|	[a-zA-Z]|	a through z or A through Z, inclusive (range)|
+|4|	[a-d[m-p]]	|a through d, or m through p: [a-dm-p] (union)|
+|5|	[a-z&&[def]]|	d, e, or f (intersection)|
+|6|	[a-z&&[^bc]]|	a through z, except for b and c: [ad-z] (subtraction)|
+|7|	[a-z&&[^m-p]]|	a through z, and not m through p: [a-lq-z](subtraction)|
+
+
+
+---------
+
+##### Regular Expression Character classes Example
+
+```java
+import java.util.regex.*;  
+class RegexExample3{  
+public static void main(String args[]){  
+System.out.println(Pattern.matches("[amn]", "abcd"));//false (not a or m or n)  
+System.out.println(Pattern.matches("[amn]", "a"));//true (among a or m or n)  
+System.out.println(Pattern.matches("[amn]", "ammmna"));//false (m and a comes more than once)  
+}
+} 
+```
+
+##### Regex Quantifiers
+
+The quantifiers specify the number of occurrences of a character.
+
+|Regex	|Description|
+|---------|---------|
+|X?	|X occurs once or not at all|
+|X+	|X occurs once or more times|
+|X*	|X occurs zero or more times|
+|X{n}	|X occurs n times only|
+|X{n,}|	X occurs n or more times|
+|X{y,z}	|X occurs at least y times but less than z times|
+
+
+---------
+
+##### Example
+```java
+    import java.util.regex.*;  
+    class RegexExample4{  
+    public static void main(String args[]){  
+    System.out.println("? quantifier ....");  
+    System.out.println(Pattern.matches("[amn]?", "a"));//true (a or m or n comes one time)  
+    System.out.println(Pattern.matches("[amn]?", "aaa"));//false (a comes more than one time)  
+    System.out.println(Pattern.matches("[amn]?", "aammmnn"));//false (a m and n comes more than one time)  
+    System.out.println(Pattern.matches("[amn]?", "aazzta"));//false (a comes more than one time)  
+    System.out.println(Pattern.matches("[amn]?", "am"));//false (a or m or n must come one time)  
+      
+    System.out.println("+ quantifier ....");  
+    System.out.println(Pattern.matches("[amn]+", "a"));//true (a or m or n once or more times)  
+    System.out.println(Pattern.matches("[amn]+", "aaa"));//true (a comes more than one time)  
+    System.out.println(Pattern.matches("[amn]+", "aammmnn"));//true (a or m or n comes more than once)  
+    System.out.println(Pattern.matches("[amn]+", "aazzta"));//false (z and t are not matching pattern)  
+      
+    System.out.println("* quantifier ....");  
+    System.out.println(Pattern.matches("[amn]*", "ammmna"));//true (a or m or n may come zero or more times)  
+      
+    }
+}  
+```
+
+----------
+
+##### Regex Metacharacters
+
+The regular expression metacharacters work as shortcodes.
+
+|Regex	|Description|
+|------|--------|
+|.	|Any character (may or may not match terminator)|
+|\d	|Any digits, short of [0-9]|
+|\D	|Any non-digit, short for [^0-9]|
+|\s	|Any whitespace character, short for [\t\n\x0B\f\r]|
+|\S	|Any non-whitespace character, short for [^\s]|
+|\w	|Any word character, short for [a-zA-Z_0-9]|
+|\W	|Any non-word character, short for [^\w]|
+|\b	|A word boundary|
+|\B|	A non word boundary|
+
+
+---------
+
+##### Regular Expression Metacharacters Example
+
+```java
+    import java.util.regex.*;  
+    class RegexExample5{  
+    public static void main(String args[]){  
+    System.out.println("metacharacters d....");\\d means digit  
+      
+    System.out.println(Pattern.matches("\\d", "abc"));//false (non-digit)  
+    System.out.println(Pattern.matches("\\d", "1"));//true (digit and comes once)  
+    System.out.println(Pattern.matches("\\d", "4443"));//false (digit but comes more than once)  
+    System.out.println(Pattern.matches("\\d", "323abc"));//false (digit and char)  
+      
+    System.out.println("metacharacters D....");\\D means non-digit  
+      
+    System.out.println(Pattern.matches("\\D", "abc"));//false (non-digit but comes more than once)  
+    System.out.println(Pattern.matches("\\D", "1"));//false (digit)  
+    System.out.println(Pattern.matches("\\D", "4443"));//false (digit)  
+    System.out.println(Pattern.matches("\\D", "323abc"));//false (digit and char)  
+    System.out.println(Pattern.matches("\\D", "m"));//true (non-digit and comes once)  
+      
+    System.out.println("metacharacters D with quantifier....");  
+    System.out.println(Pattern.matches("\\D*", "mak"));//true (non-digit and may come 0 or more times)  
+      
+    }
+}  
+```
+
+
+
+