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Java Regular Expressions


What is a Regular Expression?

A regular expression is a sequence of characters that forms a search pattern. When you search for data in a text, you can use this search pattern to describe what you are searching for.

A regular expression can be a single character, or a more complicated pattern.

Regular expressions can be used to perform all types of text search and text replace operations.

Java does not have a built-in Regular Expression class, but we can import the java.util.regex package to work with regular expressions. The package includes the following classes:

  • Pattern Class - Defines a pattern (to be used in a search)
  • Matcher Class - Used to search for the pattern
  • PatternSyntaxException Class - Indicates syntax error in a regular expression pattern

Example

Find out if there are any occurrences of the word "w3schools" in a sentence:

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class MyClass {
  public static void main(String[] args) {
    Pattern pattern = Pattern.compile("w3schools", Pattern.CASE_INSENSITIVE);
    Matcher matcher = pattern.matcher("Visit W3Schools!");
    boolean matchFound = matcher.find();
    if(matchFound) {
      System.out.println("Match found");
    } else {
      System.out.println("Match not found");
    }
  }
}
// Outputs Match found

Run example »

Example Explained

In this example, The word "w3schools" is being searched for in a sentence.

First, the pattern is created using the Pattern.compile() method. The first parameter indicates which pattern is being searched for and the second parameter has a flag to indicates that the search should be case-insensitive. The second parameter is optional.

The matcher() method is used to search for the pattern in a string. It returns a Matcher object which contains information about the search that was performed.

The find() method returns true if the pattern was found in the string and false if it was not found.


Flags

Flags in the compile() method change how the search is performed. Here are a few of them:

  • Pattern.CASE_INSENSITIVE - The case of letters will be ignored when performing a search.
  • Pattern.LITERAL - Special characters in the pattern will not have any special meaning and will be treated as ordinary characters when performing a search.
  • Pattern.UNICODE_CASE - Use it together with the CASE_INSENSITIVE flag to also ignore the case of letters outside of the English alphabet

Regular Expression Patterns

The first parameter of the Pattern.compile() method is the pattern. It describes what is being searched for.

Brackets are used to find a range of characters:

Expression Description
[abc] Find one character from the options between the brackets
[^abc] Find one character NOT between the brackets
[0-9] Find one character from the range 0 to 9

Metacharacters

Metacharacters are characters with a special meaning:

Metacharacter Description
| Find a match for any one of the patterns separated by | as in: cat|dog|fish
. Find just one instance of any character
^ Finds a match as the beginning of a string as in: ^Hello
$ Finds a match at the end of the string as in: World$
\d Find a digit
\s Find a whitespace character
\b Find a match at the beginning of a word like this: \bWORD, or at the end of a word like this: WORD\b
\uxxxx Find the Unicode character specified by the hexadecimal number xxxx

Quantifiers

Quantifiers define quantities:

Quantifier Description
n+ Matches any string that contains at least one n
n* Matches any string that contains zero or more occurrences of n
n? Matches any string that contains zero or one occurrences of n
n{x} Matches any string that contains a sequence of X n's
n{x,y} Matches any string that contains a sequence of X to Y n's
n{x,} Matches any string that contains a sequence of at least X n's

Note: If your expression needs to search for one of the special characters you can use a backslash ( \ ) to escape them. In Java, backslashes in strings need to be escaped themselves, so two backslashes are needed to escape special characters. For example, to search for one or more question marks you can use the following expression: "\\?"