- Java Basic
- Java Object Oriented
- Java Advanced
- Java Regular Expressions
JJava provides the java.util.regex package for pattern matching with regular expressions. Java regular expressions are very similar to the Perl programming language and very easy to learn.
A regular expression is a special sequence of characters that helps you match or find other strings or sets of strings, using a specialized syntax held in a pattern. They can be used to search, edit, or manipulate text and data.
The java.util.regex package primarily consists of the following three classes:
- Pattern Class: A Pattern object is a compiled representation of a regular expression. The Pattern class provides no public constructors. To create a pattern, you must first invoke one of its public static compile methods, which will then return a Pattern object. These methods accept a regular expression as the first argument.
- Matcher Class: A Matcher object is the engine that interprets the pattern and performs match operations against an input string. Like the Pattern class, Matcher defines no public constructors. You obtain a Matcher object by invoking the matcher method on a Pattern object.
- PatternSyntaxException: A PatternSyntaxException object is an unchecked exception that indicates a syntax error in a regular expression pattern.
Capturing Groups:
Capturing groups are a way to tresat multiple characters as a single unit. They are created by placing the characters to be grouped inside a set of parentheses. For example, the regular expression (dog) creates a single group containing the letters "d", "o", and "g".
Capturing groups are numbered by counting their opening parentheses from left to right. In the expression ((A)(B(C))), for example, there are four such groups:
- ((A)(B(C)))
- (A)
- (B(C))
- (C)
To find out how many groups are present in the expression, call the groupCount method on a matcher object. The groupCount method returns an int showing the number of capturing groups present in the matcher's pattern.
There is also a special group, group 0, which always represents the entire expression. This group is not included in the total reported by groupCount.
Example
Output
Regular Expression Syntax:
Here is the table listing down all the regular expression metacharacter syntax available in Java:
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Methods of the Matcher Class:
Here is the lists of useful instance methods:
Index Methods:
Index methods provide useful index values that show precisely where the match was found in the input string:
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Study Methods:
Study methods review the input string and return a boolean indicating whether or not the pattern is found:
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Replacement Methods:
Replacement methods are useful methods for replacing text in an input string:
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The start and end Methods:
Following is the example that counts the number of times the word "cats" appears in the input string:
Example
Output
You can see that this example uses word boundaries to ensure that the letters "c" "a" "t" are not merely a substring in a longer word. It also gives some useful information about where in the input string the match has occurred. The start method returns the start index of the subsequence captured by the given group during the previous match operation, and end returns the index of the last character matched, plus one.
The matches and lookingAt Methods:
The matches and lookingAt methods both attempt to match an input sequence against a pattern. The difference, however, is that matches requires the entire input sequence to be matched, while lookingAt does not.
Both methods always start at the beginning of the input string. Here is the example explaining the functionality:
Example
Output
