Java World: Java Regex Tutorial

1. Regular Expressions

1.1. Overview

A regular expression defines a search pattern for strings. This pattern may match one or several times or not at all for a given string. The abbreviation for regular expression is regex.
A simple example for a regular expression is a (literal) string. For example the Hello World regex will match the "Hello World" string.
.. (dot) is another example for an regular expression. .. matches any single character; it would match for example "a" or "z" or "1".

1.2. Usage

Regular expressions can be used to search, edit and manipulate text.
Regular expressions are supported by most programming languages, e.g. Java, Perl, Groovy, etc.
Unfortunately each language supports regular expressions slightly different.
If a regular expression is used to analyse or modify a text, this process is called The regular expression is applied to the text.
The pattern defined by the regular expression is applied on the string from left to right. Once a source character has been used in a match, it cannot be reused. For example the regex "aba" will match "ababababa" only two times (aba_aba__).

2. Prerequisites

Some of the following examples use JUnit to validate the result. You should be able to adjust them in case if you do not want to use JUnit. To learn about JUnit please see JUnit Tutorial .

3. Regular Expressions

The following is an overview of regular expressions. This chapter is supposed to be a references for the different regex elements.

3.1. Common matching symbols

Table 1.

Regular Expression	Description
`.`	Matches any sign
`^regex`	regex must match at the beginning of the line
`regex$`	Finds regex must match at the end of the line
`[abc]`	Set definition, can match the letter a or b or c
`[abc][vz]`	Set definition, can match a or b or c followed by either v or z
`[^abc]`	When a "^" appears as the first character inside [] when it negates the pattern. This can match any character except a or b or c
`[a-d1-7]`	Ranges, letter between a and d and figures from 1 to 7, will not match d1
`X\|Z`	Finds X or Z
`XZ`	Finds X directly followed by Z
`$`	Checks if a line end follows

3.2. Metacharacters

The following metacharacters have a pre-defined meaning and make certain common pattern easier to use, e.g. \d instead of [0..9].

Table 2.

Regular Expression	Description
`\d`	Any digit, short for [0-9]
`\D`	A non-digit, short for [^0-9]
`\s`	A whitespace character, short for [ \t\n\x0b\r\f]
`\S`	A non-whitespace character, for short for [^\s]
`\w`	A word character, short for [a-zA-Z_0-9]
`\W`	A non-word character [^\w]
`\S+`	Several non-whitespace characters
`\b`	Matches a word boundary. A word character is [a-zA-Z0-9_] and \b matches its bounderies.

3.3. Quantifier

A quantifier defines how often an element can occur. The symbols ?, *, + and {} define the quantity of the regular expressions

Table 3.

Regular Expression	Description	Examples
`*`	Occurs zero or more times, is short for {0,}	X* - Finds no or several letter X, .* - any character sequence
`+`	Occurs one or more times, is short for {1,}	X+ - Finds one or several letter X
`?`	Occurs no or one times, ? is short for {0,1}	X? -Finds no or exactly one letter X
`{X}`	Occurs X number of times, {} describes the order of the preceding liberal	\d{3} - Three digits, .{10} - any character sequence of length 10
`{X,Y}`	Occurs between X and Y times,	\d{1,4}- \d must occur at least once and at a maximum of four
`*?`	? after a qualifier makes it a "reluctant quantifier", it tries to find the smallest match.

3.4. Grouping and Backreference

You can group parts of your regular expression. In your pattern you group elements via round brackets, e.g. "()". This allows you to assign a repetition operator the a complete group.

In addition these groups also create a backreference to the part of the regular expression. This captures the group. A backreference stores the part of the String which matched the group. This allows you to use this part in the replacement.

Via the $ you can refer to a group. $1 is the first group, $2 the second, etc.

Lets for example assume you want to replace all whitespace between a letter followed by a point or a comma. This would involve that the point or the comma is part of the pattern. Still it should be included in the result

// Removes whitespace between a word character and . or ,
String pattern = "(\\w)(\\s+)([\\.,])";
System.out.println(EXAMPLE_TEST.replaceAll(pattern, "</code>$3"));

3.5. Negative Lookahead

Negative Lookahead provide the possibility to exclude a pattern. With this you can say that a string should not be followed by another string.

Negative Lookaheads are defined via (?!pattern). For example the following will match a if a is not followed by b.

a(?!b)

3.6. Backslashes in Java

The backslash is an escape character in Java Strings. e.g. backslash has a predefined meaning in Java. You have to use "\\" to define a single backslash. If you want to define "\w" then you must be using "\\w" in your regex. If you want to use backslash you as a literal you have to type \\\\ as \ is also a escape character in regular expressions.

4. Using Regular Expressions with String.matches()

4.1. Overview

Strings in Java have build in support for regular expressions. Strings have three build in methods for regular expressions, e.g. matches(), split()), replace(). .
These methods are not optimized for performance. We will later use classes which are optimized for performance.

Table 4.

Method	Description
s.matches("regex")	Evaluates if "regex" matches s. Returns only true if the WHOLE string can be matched
s.split("regex")	Creates array with substrings of s divided at occurance of "regex". "regex" is not included in the result.
s.replace("regex"), "replacement"	Replaces "regex" with "replacement

Create for the following example the Java project de.vogella.regex.test

public class RegexTestStrings {
  public static final String EXAMPLE_TEST = "This is my small example "
      + "string which I'm going to " + "use for pattern matching.";

  public static void main(String[] args) {
    System.out.println(EXAMPLE_TEST.matches("\\w.*"));
    String[] splitString = (EXAMPLE_TEST.split("\\s+"));
    System.out.println(splitString.length);// Should be 14
    for (String string : splitString) {
      System.out.println(string);
    }
    // Replace all whitespace with tabs
    System.out.println(EXAMPLE_TEST.replaceAll("\\s+", "\t"));
  }
}

4.2. Examples

The following class gives several examples for the usage of regular expressions with strings. See the comment for the purpose.
If you want to test these examples, create for the Java project de.vogella.regex.string.

public class StringMatcher {
// Returns true if the string matches exactly "true"
public boolean isTrue(String s){
    return s.matches("true");
}
// Returns true if the string matches exactly "true" or "True"
public boolean isTrueVersion2(String s){
    return s.matches("[tT]rue");
}

// Returns true if the string matches exactly "true" or "True"
// or "yes" or "Yes"
public boolean isTrueOrYes(String s){
    return s.matches("[tT]rue|[yY]es");
}

// Returns true if the string contains exactly "true"
public boolean containsTrue(String s){
    return s.matches(".*true.*");
}

// Returns true if the string contains of three letters
public boolean isThreeLetters(String s){
    return s.matches("[a-zA-Z]{3}");
    // Simpler from for
//    return s.matches("[a-Z][a-Z][a-Z]");
}

// Returns true if the string does not have a number at the beginning
public boolean isNoNumberAtBeginning(String s){
    return s.matches("^[^\\d].*");
}
// Returns true if the string contains a arbitrary number of characters except b
public boolean isIntersection(String s){
    return s.matches("([\\w&&[^b]])*");
}
// Returns true if the string contains a number less then 300
public boolean isLessThenThreeHundret(String s){
    return s.matches("[^0-9]*[12]?[0-9]{1,2}[^0-9]*");
}

}

5. Pattern and Matcher

For advanced regular expressions the java.util.regex.Pattern and java.util.regex.Matcher classes are used.
You first create a Pattern object which defines the regular expression. This Pattern object allows you to create a Matcher object for a given string. This Matcher object then allows you to do regex operations on a String.

import java.util.regex.Matcher;
import java.util.regex.Pattern; public class RegexTestPatternMatcher {
public static final String EXAMPLE_TEST = "This is my small example string which I'm going to use for pattern matching.";

public static void main(String[] args) {
    Pattern pattern = Pattern.compile("\\w+");
    // In case you would like to ignore case sensitivity you could use this
    // statement
    // Pattern pattern = Pattern.compile("\\s+", Pattern.CASE_INSENSITIVE);
    Matcher matcher = pattern.matcher(EXAMPLE_TEST);
    // Check all occurance
    while (matcher.find()) {
      System.out.print("Start index: " + matcher.start());
      System.out.print(" End index: " + matcher.end() + " ");
      System.out.println(matcher.group());
    }
    // Now create a new pattern and matcher to replace whitespace with tabs
    Pattern replace = Pattern.compile("\\s+");
    Matcher matcher2 = replace.matcher(EXAMPLE_TEST);
    System.out.println(matcher2.replaceAll("\t"));
}
}

6. Java Regex Examples

The following lists typical examples for the usage of regular expressions. I hope you find similarities to your examples.

6.1. Or

Task: Write a regular expression which matches a text line if this text line contains either the word "Joe" or the word "Jim" or both.
Create a project de.vogella.regex.eitheror and the following class.

import org.junit.Test;

import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

public class EitherOrCheck {
@Test
public void testSimpleTrue() {
    String s = "humbapumpa jim";
    assertTrue(s.matches(".*(jim|joe).*"));
    s = "humbapumpa jom";
    assertFalse(s.matches(".*(jim|joe).*"));
    s = "humbaPumpa joe";
    assertTrue(s.matches(".*(jim|joe).*"));
    s = "humbapumpa joe jim";
    assertTrue(s.matches(".*(jim|joe).*"));
}
}

6.2. Phone number

Task: Write a regular expression which matches any phone number.
A phone number in this example consists either out of 7 numbers in a row or out of 3 number a (white)space or a dash and then 4 numbers.

import org.junit.Test;

import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue; public class CheckPhone {

@Test
public void testSimpleTrue() {
    String pattern = "\\d\\d\\d([,\\s])?\\d\\d\\d\\d";
    String s= "1233323322";
    assertFalse(s.matches(pattern));
    s = "1233323";
    assertTrue(s.matches(pattern));
    s = "123 3323";
    assertTrue(s.matches(pattern));
}
}

6.3. Check for a certain number range

The following example will check if a text contains a number with 3 digits.
Create the Java project "de.vogella.regex.numbermatch" and the following class

import org.junit.Test;

import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

public class CheckNumber {

@Test
public void testSimpleTrue() {
    String s= "1233";
    assertTrue(test(s));
    s= "0";
    assertFalse(test(s));
    s = "29 Kasdkf 2300 Kdsdf";
    assertTrue(test(s));
    s = "99900234";
    assertTrue(test(s));
}

public static boolean test (String s){
    Pattern pattern = Pattern.compile("\\d{3}");
    Matcher matcher = pattern.matcher(s);
    if (matcher.find()){
      return true;
    }
    return false;
}

}

6.4. Building a link checker

The following example allows you to extract all valid links from a webpage. It does not consider links with start with "javascript:" or "mailto:".

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class LinkGetter {
private Pattern htmltag;
private Pattern link;
private final String root;

public LinkGetter(String root) {
    this.root = root;
    htmltag = Pattern.compile("<a\\b[^>]*href=\"[^>]*>(.*?)</a>");
    link = Pattern.compile("href=\"[^>]*\">");
}

public List<String> getLinks(String url) {
    List<String> links = new ArrayList<String>();
    try {
      BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(new URL(url).openStream()));
      String s;
      StringBuilder builder = new StringBuilder();
      while ((s = bufferedReader.readLine()) != null) {
        builder.append(s);
      }

      Matcher tagmatch = htmltag.matcher(builder.toString());
      while (tagmatch.find()) {
        Matcher matcher = link.matcher(tagmatch.group());
        matcher.find();
        String link = matcher.group().replaceFirst("href=\"", "")
            .replaceFirst("\">", "");
        if (valid(link)) {
          links.add(makeAbsolute(url, link));
        }
      }
    } catch (MalformedURLException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    }
    return links;
}

private boolean valid(String s) {
    if (s.matches("javascript:.*|mailto:.*")) {
      return false;
    }
    return true;
}

private String makeAbsolute(String url, String link) {
    if (link.matches("http://.*")) {
      return link;
    }
    if (link.matches("/.*") && url.matches(".*$[^/]")) {
      return url + "/" + link;
    }
    if (link.matches("[^/].*") && url.matches(".*[^/]")) {
      return url + "/" + link;
    }
    if (link.matches("/.*") && url.matches(".*[/]")) {
      return url + link;
    }
    if (link.matches("/.*") && url.matches(".*[^/]")) {
      return url + link;
    }
    throw new RuntimeException("Cannot make the link absolute. Url: " + url
        + " Link " + link);
}
}

6.5. Finding duplicated words

The regular expression \b(\w+) \1\b matches duplicated words. The (?!-in)\b(\w+) \1\b finds duplicate words if they do not start with "-in".

Java World

Tuesday, October 16, 2012

Java Regex Tutorial