Java Regular Expression Examples
Java Regular Expression Examples
Java 8 stream and regular expression examples.
1. String.matches(regex)
1.1 This example, check if the string is a number.
JavaRegEx1.java
package com.mkyong.regex;
import java.util.Arrays;
import java.util.List;
public class JavaRegEx1 {
public static void main(String[] args) {
List<String> numbers = Arrays.asList("1", "20", "A1", "333", "A2A211");
for (String number : numbers) {
if (number.matches("\\d+")) {
System.out.println(number); // 1, 20, 333
}
}
// Java 8 stream example
numbers.stream()
.filter(x -> x.matches("\\d+"))
.forEach(System.out::println);
}
}
Output
1
20
333
1
20
333
2. String.replaceAll(regex, replacement)
2.1 This example replaces all digits with
#
JavaRegEx2.java
package com.mkyong.regex;
import java.util.Arrays;
import java.util.List;
public class JavaRegEx2 {
public static void main(String[] args) {
List<String> numbers = Arrays.asList("1", "20", "A1", "333", "A2A211");
for (String number : numbers) {
System.out.println(number.replaceAll("\\d", "#"));
}
// Java 8 stream example
numbers.stream()
.map(x -> x.replaceAll("\\d", "#"))
.forEach(System.out::println);
}
}
Output
#
##
A#
###
A#A###
#
##
A#
###
A#A###
3. Pattern and Matcher
3.1 Find all digits from a list of String.
JavaRegEx3.java
package com.mkyong.regex;
import java.util.Arrays;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class JavaRegEx3 {
public static void main(String[] args) {
List<String> numbers = Arrays.asList("1", "20", "A1", "333", "A2A211");
Pattern pattern = Pattern.compile("\\d+");
for (String number : numbers) {
Matcher matcher = pattern.matcher(number);
while (matcher.find()) {
System.out.println(matcher.group(0));
}
}
}
}
Output
1
20
1
333
2
211
3.2 For Java 8 stream, first we try to convert like this:
numbers.stream()
.map(x -> pattern.matcher(x))
.filter(Matcher::find) // A2A211, will it loop?
.map(x -> x.group())
.forEach(x -> System.out.println(x));
Output, the last 211 is missing?
1
20
1
333
2
The stream can’t loop the
.filter to get all the groups, we need a hack with a custom Spliterators
JavaRegEx4.java
package com.mkyong.regex;
import java.util.Arrays;
import java.util.List;
import java.util.Spliterators;
import java.util.function.Consumer;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.util.stream.StreamSupport;
public class JavaRegEx4 {
public static void main(String[] args) {
List<String> numbers = Arrays.asList("1", "20", "A1", "333", "A2A211");
Pattern pattern = Pattern.compile("\\d+");
numbers.stream()
.flatMap(x ->
StreamSupport.stream(new MatchItr(pattern.matcher(x)), false))
.forEach(x -> System.out.println(x));
}
final static class MatchItr extends Spliterators.AbstractSpliterator<String> {
private final Matcher matcher;
MatchItr(Matcher m) {
super(m.regionEnd() - m.regionStart(), ORDERED | NONNULL);
matcher = m;
}
public boolean tryAdvance(Consumer<? super String> action) {
if (!matcher.find()) return false;
action.accept(matcher.group());
return true;
}
}
}
Output
1
20
1
333
2
211
4. Java 9, Scanner.findAll(regex)
4.1 Java 9, we can use
Scanner.findAll(regex) to return a stream of match results that match the provided regex. Scanner scan = new Scanner("A2A211");
List<String> collect = scan
.findAll("\\d+")
.map(m -> m.group())
.collect(Collectors.toList());
collect.forEach(x -> System.out.println(x));
Output
2
211
Final version, with Java 9.
JavaRegEx5.java
package com.mkyong.regex;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;
import java.util.regex.Pattern;
import java.util.stream.Collectors;
public class JavaRegEx5 {
public static void main(String[] args) {
List<String> numbers = Arrays.asList("1", "20", "A1", "333", "A2A211");
Pattern pattern = Pattern.compile("\\d+");
List<String> collect = numbers.stream()
.map(x -> new Scanner(x).findAll(pattern)
.map(m -> m.group())
.collect(Collectors.toList())
)
.flatMap(List::stream)
.collect(Collectors.toList());
collect.forEach(x -> System.out.println(x));
}
}
Output
1
20
1
333
2
211
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