Any group of strings according to a particular pattern is called regular expression eg. To represent all valid mail id, by using regular expression we can validate whether the given mail id is valid or not. To represent all valid java identifiers

The main application areas of Regular Expression are -

• Implementation of validation mechanism
• Developing pattern matching applications
• Developing translators like compilers and interpreters
• Designing digital circuits
• Developing communication protocol like TCP, UDP

Pattern Class

A pattern object represents compiled version of regular expression, we can create a pattern object by using compile() of pattern class.

Pattern p = Pattern.compile(String regularExpression);

Matcher Class

A matcher object can be used to match character sequence against a regular expression. we can create a matcher object by using matcher() of Pattern class

Matcher m = p.matcher(String target);

Important methods of Matcher Class

boolean find() - To find next match and if it is not available return true otherwise return false

int start() - Returns start index of the match

int end() - Returns end index of the match

String group() - Returns the matched pattern

Character Classes

[a-z] Any lower case alphabet symbol

[A-Z] Any upper case alphabet symbol

[a-z-A-Z] Any alphabet symbol

[0-9] Any digit 0 to 9

[abc] either a or b or c

[^abc] Except a or b or c

[0-9-a-z-A-Z] Any Alpha Numeric Character

Predefined Character Classes

Space Character \s

[0-9] \d

[0-9-a-z-A-Z] \w

AnyCharacter .

Quantifiers - To specify number of characters to match eg.

a Exactly one a

a+ At least one a

a* Any number of a's

a? Atmost one a

Split() method

Pattern class contains method to split given String according to a regular expression. String class also contains split() to split the given string against regular expression.

Note: Pattern class split() can take target String as argument where as String class split() can take regular expression as argument

StringTokenizer - class presenting is java.util package used to divide the target String into Stream of tokens.