java.util.regex.PatternSyntaxException: Illegal repetition near index

The java.util.regex.PatternSyntaxException: Illegal repetition near index 0 exception occurs when the regular expression special characters such as $ { } are used inside the java search string. When processing in the regular expression, the special characters have significant meaning. It is necessary to add a special character with an escape to the search. If the special characters are used with out an escape character, the error java.util.regex.PatternSyntaxException: Illegal repetition near index 0 is thrown out.

The regular expression engine recognises the character { as it is about to start a repeat predictor, such as {1,5}, which is 1 to 5 times the previous token. For example, {a} is illegal as the a number must be followed by the character ‘{‘ , thus the exception Illegal repetition near index will be thrown.

The special characters such as $, {, }, [, ] has significant meaning in the regular expression. Curly brackets define the character group found in the given string. The $ determines the character’s end of the line. If the special characters must be escape to search the character. Otherwise the regular expression will throw exception.

Exception

The java exception java.util.regex.PatternSyntaxException: Illegal repetition near index 0 will display the stack trace as shown below.

Exception in thread "main" java.util.regex.PatternSyntaxException: Illegal repetition near index 0
${name}
^
	at java.util.regex.Pattern.error(Pattern.java:1955)
	at java.util.regex.Pattern.closure(Pattern.java:3157)
	at java.util.regex.Pattern.sequence(Pattern.java:2134)
	at java.util.regex.Pattern.expr(Pattern.java:1996)
	at java.util.regex.Pattern.compile(Pattern.java:1696)
	at java.util.regex.Pattern.<init>(Pattern.java:1351)
	at java.util.regex.Pattern.compile(Pattern.java:1028)
	at java.lang.String.replaceAll(String.java:2223)
	at com.yawintutor.StringReplaceAll.main(StringReplaceAll.java:10)

How to recreate this exception

In the regular expression, add substitute variable with $ and curly brackets { }. These characters have special meaning with in the regular expression. The search string containing the special characters will throw the exception as shown below.

package com.yawintutor;

public class StringReplaceAll {
	public static void main(String[] args) {
		String str = "Welcome ${name}";
		String pattern = "${name}";
		String str2;

		str2 = str.replaceAll(pattern, "");

		System.out.println("Given  String : " + str);
		System.out.println("Output String : " + str2);
	}
}

Output

Exception in thread "main" java.util.regex.PatternSyntaxException: Illegal repetition near index 0
${name}
^
	at java.util.regex.Pattern.error(Pattern.java:1955)
	at java.util.regex.Pattern.closure(Pattern.java:3157)
	at java.util.regex.Pattern.sequence(Pattern.java:2134)
	at java.util.regex.Pattern.expr(Pattern.java:1996)
	at java.util.regex.Pattern.compile(Pattern.java:1696)
	at java.util.regex.Pattern.<init>(Pattern.java:1351)
	at java.util.regex.Pattern.compile(Pattern.java:1028)
	at java.lang.String.replaceAll(String.java:2223)
	at com.yawintutor.StringReplaceAll.main(StringReplaceAll.java:10)

Root Cause

The special characters such as $ { } [ ] is used to identify a group of characters within a given String. If special characters are used to match characters in a string, Java attempts to validate them based on the unique function of the character that generates the exception. In order to search for a character in a given string, special characters must escape.

Solution 1

In order to search for a character in a given string, special characters must escape. The special characters in the regular expression will be searched as a character if it is marked with escape character. The example below shows how to add escape in the regular expression

package com.yawintutor;

public class StringReplaceAll {
	public static void main(String[] args) {
		String str = "Welcome ${name}";
		String pattern = "\\$\\{name\\}";
		String str2;

		str2 = str.replaceAll(pattern, "");

		System.out.println("Given  String : " + str);
		System.out.println("Output String : " + str2);
	}
}

Output

Given  String : Welcome ${name}
Output String : Welcome 

Solution 2

The character { should be used to start a repeat predictor. A number must be followed by the character {. For example, {1,5} which is 1 to 5 times the previous token.

package com.yawintutor;

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

public class StringReplaceAll {
	public static void main(String[] args) {
		String str = "Welcome ${name}";
		String pattern = "e{1,3}";
		String str2;

		str2 = str.replaceAll(pattern, "");

		System.out.println("Given  String : " + str);
		System.out.println("Output String : " + str2);
	}
}

Output

Given  String : Welcome ${name}
Output String : Wlcom ${nam}