Java RegEx - How to Escape and Match Bracket

Java RegEx – How to Escape and Match bracket example shows how to escape and match square bracket and round bracket (parentheses) literally in Java regular expression pattern.

How to match a square bracket using regex in Java?

The square brackets in Java’s regular expression have a special meaning. They are used to define a character class. See the below-given regex pattern for example.

[^0-9]

This pattern matches with any character that is not a digit between 0 to 9.

package com.javacodeexamples.regex;

public class RegExEscapeBracket {

public static void main(String[] args) {

String str = "123abc456";

System.out.println( str.replaceAll("[^0-9]", "") );

}

Output

123456

As you can see from the output, all the non-digit characters were removed from the string.

Since the square brackets are used to define a character class in a regex, we cannot directly mention that when we want to match it literally. See the below example.

package com.javacodeexamples.regex;

public class RegExEscapeBracket {

public static void main(String[] args) {

String str = "[One],[two]";

str = str.replaceAll("[", "");

System.out.println(str);

}

Output

Exception in thread "main" java.util.regex.PatternSyntaxException: Unclosed character class near index 0

[

at java.util.regex.Pattern.error(Unknown Source)

at java.util.regex.Pattern.clazz(Unknown Source)

at java.util.regex.Pattern.sequence(Unknown Source)

at java.util.regex.Pattern.expr(Unknown Source)

at java.util.regex.Pattern.compile(Unknown Source)

at java.util.regex.Pattern.<init>(Unknown Source)

at java.util.regex.Pattern.compile(Unknown Source)

at java.lang.String.replaceAll(Unknown Source)

As you can see from the output, the code throws java.util.regex.PatternSyntaxException: Unclosed character class near index exception, since it was expecting a character class when it encountered the opening square bracket.

In order to match the square bracket literally, we need to escape it like “\\[” as given below.

String str = "[One],[two]";

str = str.replaceAll("\\[", "");

System.out.println(str);

Output

One],two]

How to match a round bracket or parentheses using regex in Java?

Just like the square bracket, a round bracket has also a special meaning in the regex. It is used to define a regex group which we can later extract.

String str = "(One), (Two)";

str = str.replaceAll("(", "");

System.out.println(str);

Output

Exception in thread "main" java.util.regex.PatternSyntaxException: Unclosed group near index 1

(

at java.util.regex.Pattern.error(Unknown Source)

at java.util.regex.Pattern.accept(Unknown Source)

at java.util.regex.Pattern.group0(Unknown Source)

at java.util.regex.Pattern.sequence(Unknown Source)

at java.util.regex.Pattern.expr(Unknown Source)

at java.util.regex.Pattern.compile(Unknown Source)

at java.util.regex.Pattern.<init>(Unknown Source)

at java.util.regex.Pattern.compile(Unknown Source)

at java.lang.String.replaceAll(Unknown Source)

An exception is thrown complaining about an unclosed group since it was expecting a group to be defined using the parentheses. This time the exception is java.util.regex.PatternSyntaxException: Unclosed group near index 1. Just like the square bracket, we also need to escape the round bracket like “\\(” in order to match it literally.

String str = "(One), (Two)";

str = str.replaceAll("\\(", "");

System.out.println(str);

Output

1	One), Two)

We need to escape all metacharacters in such a way wherever we are using the regex, for example, in the string split method, string replaceAll method, and while compiling a pattern.

If you want to learn more about the regex, please visit the Java RegEx tutorial.

Please let me know your views in the comments section below.