Java Regex Tutorial
Copyright © 2007, 2008, 2009, 2010, 2011, 2012, 2013 Lars Vogel
| Revision History |
|---|
| Revision 0.1 | 07.12.2007 | LarsVogel | created |
| Revision 0.2 - 2.3 | 30.12.2008 - 09.10.2013 | LarsVogel | bug fixes and enhancements |
Java and Regular Expressions
This tutorial introduces the usage of regular expressions and describes their implementation in Java. It also provides several Java regular expression examples.
A regular expression defines a search pattern for strings. Regular expressions can be used to search, edit and manipulate text. The pattern defined by the regular expression may match one or several times or not at all for a given string.
The abbreviation for regular expression is regex.
The process of analyzing or modifying a text with a regex is called: The regular expression is applied to the text (string) .
The pattern defined by the regex is applied on the text 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__).
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 a regular expression. A dot matches any single character; it would match, for example, "a" or "z" or "1".
1.2. Support for regular expressions in programming languages
Regular expressions are supported by most programming languages, e.g., Java, Perl, Groovy, etc. Unfortunately each language supports regular expressions slightly different.
This tutorial describes the usage of regular expression within the Java programming language and within the Eclipse IDE.
The following tutorial assumes that you have basic knowledge of the Java programming language.
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. Rules of writing regular expressions
The following description is an overview of available meta characters which can be used in 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 character |
^regex | Finds regex that must match at the beginning of the line. |
regex$ | Finds regex that 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 caret appears as the first character inside square brackets, it negates the pattern. This can match any character except a or b or c. |
[a-d1-7] | Ranges: matches a letter between a and d and figures from 1 to 7, but not d1. |
X|Z | Finds X or Z. |
XZ | Finds X directly followed by Z. |
$ | Checks if a line end follows. |
The following metacharacters have a pre-defined meaning and make certain common patterns 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, 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 where a word character is [a-zA-Z0-9_]. |
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,
.* finds 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} searches for three digits, .{10} for any character sequence of length 10. |
{X,Y} | Occurs between X and Y times, | \d{1,4} means \d must occur at least once and at a maximum of four. |
*? | ? after a quantifier 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 with round brackets, e.g., (). This allows you to assign a repetition operator to 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.
Let's, 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.
String pattern = "(\\w)(\\s+)([\\.,])";
System.out.println(EXAMPLE_TEST.replaceAll(pattern, "$1$3"));
This example extracts the text between a title tag.
pattern = "(?i)(<title.*?>)(.+?)(</title>)";
String updated = EXAMPLE_TEST.replaceAll(pattern, "$2");
Negative Lookahead provides 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)
The backslash \ is an escape character in Java Strings. That means backslash has a predefined meaning in Java. You have to use double backslash \\ 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 as a literal, you have to type \\\\ as\ is also an escape character in regular expressions.
4. Using Regular Expressions with String.matches()
Strings in Java have built-in support for regular expressions. Strings have three built-in methods for regular expressions, i.e., 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 an array with substrings of s divided at occurrence 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.
package 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);
for (String string : splitString) {
System.out.println(string);
}
System.out.println(EXAMPLE_TEST.replaceAll("\\s+", "\t"));
}
}
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.
package de.vogella.regex.string;
public class StringMatcher {
public boolean isTrue(String s){
return s.matches("true");
}
public boolean isTrueVersion2(String s){
return s.matches("[tT]rue");
}
public boolean isTrueOrYes(String s){
return s.matches("[tT]rue|[yY]es");
}
public boolean containsTrue(String s){
return s.matches(".*true.*");
}
public boolean isThreeLetters(String s){
return s.matches("[a-zA-Z]{3}");
}
public boolean isNoNumberAtBeginning(String s){
return s.matches("^[^\\d].*");
}
public boolean isIntersection(String s){
return s.matches("([\\w&&[^b]])*");
}
public boolean isLessThenThreeHundret(String s){
return s.matches("[^0-9]*[12]?[0-9]{1,2}[^0-9]*");
}
}
And a small JUnit Test to validates the examples.
package de.vogella.regex.string;
import org.junit.Before;
import org.junit.Test;
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;
public class StringMatcherTest {
private StringMatcher m;
@Before
public void setup(){
m = new StringMatcher();
}
@Test
public void testIsTrue() {
assertTrue(m.isTrue("true"));
assertFalse(m.isTrue("true2"));
assertFalse(m.isTrue("True"));
}
@Test
public void testIsTrueVersion2() {
assertTrue(m.isTrueVersion2("true"));
assertFalse(m.isTrueVersion2("true2"));
assertTrue(m.isTrueVersion2("True"));;
}
@Test
public void testIsTrueOrYes() {
assertTrue(m.isTrueOrYes("true"));
assertTrue(m.isTrueOrYes("yes"));
assertTrue(m.isTrueOrYes("Yes"));
assertFalse(m.isTrueOrYes("no"));
}
@Test
public void testContainsTrue() {
assertTrue(m.containsTrue("thetruewithin"));
}
@Test
public void testIsThreeLetters() {
assertTrue(m.isThreeLetters("abc"));
assertFalse(m.isThreeLetters("abcd"));
}
@Test
public void testisNoNumberAtBeginning() {
assertTrue(m.isNoNumberAtBeginning("abc"));
assertFalse(m.isNoNumberAtBeginning("1abcd"));
assertTrue(m.isNoNumberAtBeginning("a1bcd"));
assertTrue(m.isNoNumberAtBeginning("asdfdsf"));
}
@Test
public void testisIntersection() {
assertTrue(m.isIntersection("1"));
assertFalse(m.isIntersection("abcksdfkdskfsdfdsf"));
assertTrue(m.isIntersection("skdskfjsmcnxmvjwque484242"));
}
@Test
public void testLessThenThreeHundred() {
assertTrue(m.isLessThenThreeHundred("288"));
assertFalse(m.isLessThenThreeHundred("3288"));
assertFalse(m.isLessThenThreeHundred("328 8"));
assertTrue(m.isLessThenThreeHundred("1"));
assertTrue(m.isLessThenThreeHundred("99"));
assertFalse(m.isLessThenThreeHundred("300"));
}
}
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.
package de.vogella.regex.test;
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+");
Matcher matcher = pattern.matcher(EXAMPLE_TEST);
while (matcher.find()) {
System.out.print("Start index: " + matcher.start());
System.out.print(" End index: " + matcher.end() + " ");
System.out.println(matcher.group());
}
Pattern replace = Pattern.compile("\\s+");
Matcher matcher2 = replace.matcher(EXAMPLE_TEST);
System.out.println(matcher2.replaceAll("\t"));
}
}
The following lists typical examples for the usage of regular expressions. I hope you find similarities to your real-world problems.
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.
package de.vogella.regex.eitheror;
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).*"));
}
}
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.
package de.vogella.regex.phonenumber;
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.
package de.vogella.regex.numbermatch;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
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 which start with "javascript:" or "mailto:".
Create a Java project called de.vogella.regex.weblinks and the following class:
package de.vogella.regex.weblinks;
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;
public LinkGetter() {
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("\">", "")
.replaceFirst("\"[\\s]?target=\"[a-zA-Z_0-9]*", "");
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 following regular expression matches duplicated words.
\b(\w+)\s+\1\b
\b is a word boundary and \1 references to the captured match of the first group, i.e., the first word.
The (?!-in)\b(\w+) \1\b finds duplicate words if they do not start with "-in".
Tip
Add (?s) to search across multiple lines.
6.6. Finding an elements which start in a new line
The following regular expression allows you to find the "title" word, in case it starts in a new line, potentially with leading spaces.
(\n\s*)title
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