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Regular Expressions in Java

What are Regular Expressions ?

• It is an API for defining String patterns that can be used for searching, manipulating and editing a string in Java.
• Email validation and passwords are few areas of strings where Regex are widely used to define the constraints.
• Regular Expressions are provided under java.util.regex package.

java.util.regex Package

This consists of 3 classes and 1 interface :

• MatchResult Interface
• Pattern Class
• Matcher Class
• PatternSyntaxException Class

MatchResult Interface

• Used to determine the result of a match operation for a regular expression.
• It must be noted that although the match boundaries, groups and group boundaries can be seen, the modification is not allowed through a MatchResult.

Methods :

Modifier and Type	Method	Description
int	end()	To return the offset after the last character matched.
int	end(int group)	To return the offset after the last character of the subsequence captured by the given group during this match.
String	group()	To return the input subsequence matched by the previous match.
String	group(int group)	To return the input subsequence captured by the given group during the previous match operation.
int	groupCount()	To return the number of capturing groups in this match result’s pattern.
int	start()	To return the start index of the match.
int	start(int group)	To return the start index of the subsequence captured by the given group during this match.

1. Pattern Class

• It is a compilation of regular expressions that can be used to define various types of patters, providing no public constructors.
• Created by invoking the compile() method which accepts a regular expression as the first argument and returns a pattern after execution.

Basic Syntax:

Pattern pattern = Pattern.compile("geeks");

Methods :

• compile(String regex): - compiles the given regular expression into a pattern.
• compile(String regex, int flags): - compiles the given regular expression into a pattern with the given flags.
• flags(): - return the pattern’s match flags.
• matcher(CharSequence input): - creates a matcher that will match the given input against the pattern.
• matches(regex, input): - compiles the given regular expression and attempts to match the given input against it.
• pattern(): - returns the regular expression from which this pattern was compiled.
• quote(String s): -returns a literal pattern String for the specified String.
• split(input): - to split the given input sequence around matches of this pattern.
• split(input, limit): - to split the given input sequence around matches of this pattern.
• toString(): - to return the string representation of the pattern.

Modifier and Type	Method	Description
static Pattern	compile(String regex)	To compile the given regular expression into a pattern.
static Pattern	compile(String regex,int flags)	To compile the given regular expression into a pattern with given flags.
int	flags()	To return this pattern’s match flags.
Matcher	matcher(CharSequence input)	To create a matcher that will match the given input against this pattern.
static boolean	matches(String regex, CharSequence input)	To compile the given regular expression and attempts to match the given input against it.
String	pattern()	To return the regular expression from which this pattern was compiled.
static String	quote(String s)	To return a literal pattern String for the specified String.
String[]	split(CharSequence input)	To split the given input sequence around matches of this pattern.
String[]	split(CharSequence input,int limit)	To split the given input sequence around matches of this pattern.
String	toString()	To return the string representation of this pattern.

2. Matcher Class

• This object is used to perform match operations for an input string in java, thus interpreting the previously explained patterns.
• This too defines no public constructors and can be implemented by invoking a matcher() on any pattern object.

Basic Syntax:

// Create a pattern to be searched 
Pattern pattern = Pattern.compile("geeks"); 

// Search above pattern in "geeksforgeeks.org" 
Matcher m = pattern.matcher("geeksforgeeks.org");

Methods :

Modifier and Type	Method	Description
boolean	find()	For searching multiple occurrences of the regular expressions in the text.
boolean	find(int start)	For searching occurrences of the regular expressions in the text starting from the given index.
int	start()	For getting the start index of a match that is being found using find() method.
int	end()	For getting the end index of a match that is being found using find() method. It returns index of character next to last matching character.
int	groupCount()	It is used to find the total number of the matched subsequence.
String	group()	It is used to find the matched subsequence.
boolean	matches()	It is used to test whether the regular expression matches the pattern.

Note: Pattern.matches() checks if the whole text matches with a pattern or not. Other methods are mainly used to find multiple occurrences of pattern in text.

3. PatternSyntaxException class

• This object of Regex is used to indicate a syntax error in a regular expression pattern and is an unchecked exception.

Methods :

Modifier and Type	Method	Description
String	getDescription()	To retrieve the description of the error.
int	getIndex()	To retrieve the error index.
String	getMessage()	To return a multiline string containing description of syntax error and its index, the erroneous regular-expression pattern, and a visual indication of the error index within the pattern.
String	getPattern()	To retrieve the erroneous regular-expression pattern.

Important Observations / Facts

1. We create a pattern object by calling Pattern.compile(), there is no constructor. compile() is a static method in Pattern class.
2. Like above, we create a Matcher object using matcher() on objects of Pattern class.
3. Pattern.matches() is also a static method that is used to check if given text as a whole matches pattern or not.
4. find() is used to find multiple occurrences of pattern in text.
5. We can split a text based on a delimiter pattern using split()

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