Regular Expressions in Java: A Quick Start Guide
Regular expressions are a type of language used to match patterns of text, often used in programming. In Java, they’re used to perform operations such as verifying user input, splitting a string, or extracting information from a larger textual source. The ability to define and use regular expressions is a powerful skill to have when developing applications in Java.
What are Regular Expressions?
Regular expressions are a kind of shorthand or mini-language used to match patterns in text. They act as a set of building blocks for developers, allowing them to express a wide range of specific conditions about what the desired text should look like. A regular expression is simply a string of characters, but when applied the expression is interpreted in a certain way to look for specific patterns in the given text. This is often called “pattern matching”.
At its simplest, a regular expression just specifies a literal character or string of characters to look for. A regular expression can also combine literals with a set of special characters to create more complicated patterns. For example, one can use the asterisk (*) to indicate any number of any character(s).
Syntax of Regular Expressions in Java
The syntax of regular expressions in Java is outlined by the Java.re Pattern class. This class is used to compile a regular expression pattern string into a Pattern object, which can then be matched against one or more text strings. The syntax supports a variety of special characters and metacharacters that are used to define the pattern:
- Backslashes (\) escape characters.
- Curly braces ({}) indicate the number of occurrences of the previous character(s) or expression.
- Square brackets ([ ]) indicate a character set.
- Asterisks (*) indicate zero or more occurrences of the previous character or expression.
Getting Started With Regular Expressions in Java
Before you can start using regular expressions in Java, you’ll need to create a Pattern object, which can be done by calling the compile() method on the Pattern class.
The compile() method takes a pattern string as a parameter, which is the regular expression string you’ll use to search through a text string. Once you have a Pattern object, you can use it to search through a text string to find matches using the matcher() method.
Examples of Using Regular Expressions in Java
To illustrate how to use regular expressions in Java, here are a couple of examples. The first example shows how you can search a text string for the pattern “Hello”:
String text = "Hello world";
Pattern pattern = Pattern.compile("Hello");
Matcher matcher = pattern.matcher(text);
boolean matches = matcher.matches();
The above code will check the text string for the literal string “Hello”, and set the matches variable to true if it finds a match. If the pattern is not found, it will set it to false.
Here’s an example of a more complicated pattern, which searches for a date in the text:
String text = "Today is 09/25/2020";
Pattern pattern = Pattern.compile("\\d{2}\\/\\d{2}\\/\\d{4}");
Matcher matcher = pattern.matcher(text);
boolean matches = matcher.matches();
In this example, the pattern looks for two digits, followed by a forward slash character, followed by two more digits, followed by a forward slash character, followed by four digits. This is the syntax for a date in the MM/DD/YYYY format.
Conclusion
Regular expressions are an invaluable tool for understanding and manipulating text. In Java, the Pattern class allows developers to compile their own expressions. By understanding how to interpret regular expression syntax and use the Pattern and Matcher classes, developers can find patterns in text or verify the format of input strings.
