Solution - java.util.regex.PatternSyntaxException: Unclosed group near index 1

The java.util.regex.PatternSyntaxException: Unclosed group near index 1 example shows the reasons for this exception. It also shows how to fix it using various solutions.

What is java.util.regex.PatternSyntaxException: Unclosed group near index and what causes it?

There are several characters in the regular expression that have a special meaning. These are called meta-characters. Two such characters are opening and closing brackets, “(” and “)”. The opening and closing brackets are used to mention the groups in the regular expression.

There are two possible reasons for this Java exception.

Reason 1: Opening & closing bracket mismatch while extracting groups

One possible reason is that the regex pattern you have specified is having an opening bracket “(” to extract or match a group but it does not have the matching closing bracket. See below given example.

//extract numbers from string

Pattern pattern = Pattern.compile("(\\d+");

Matcher matcher = pattern.matcher("I got 95 marks out of 100 marks in Java test.");

while(matcher.find()) {

System.out.println("Matched \"" + matcher.group() + "\"");

}

When you run this code, you will get an exception as given below.

Exception in thread "main" java.util.regex.PatternSyntaxException: Unclosed group near index 4

(\d+

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)

Solution:

The problem is with the pattern we have specified. The pattern “(\\d+” is missing the closing bracket. Once we specify the closing bracket in the pattern, the code runs fine without any exception.

//extract numbers from string

Pattern pattern = Pattern.compile("(\\d+)");

Matcher matcher = pattern.matcher("I got 95 marks out of 100 marks in Java test.");

while(matcher.find()) {

System.out.println("Matched \"" + matcher.group() + "\"");

}

Output

1 2	Matched "95" Matched "100"

The easiest way to identify such an issue is to count the number of opening and closing brackets. If the numbers do not match, you are missing brackets. Fix that and this exception will go away. Plus, the exception stack trace also shows where it thinks we missed the bracket.

Reason 2: You are trying to find “(” using regex but you have not escaped it

If you want to specify the meta-characters in a literal way, you have to escape such characters. Please see below given example.

Pattern pattern = Pattern.compile("Java (");

Matcher matcher = pattern.matcher("C++ (100), Java (99), C (89)");

while(matcher.find()) {

System.out.println("Matched \"" + matcher.group() + "\"");

}

Output

Exception in thread "main" java.util.regex.PatternSyntaxException: Unclosed group near index 6

Java (

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)

Solution:

The above code tried to find the string “Java (” in a literal way. But since the opening bracket is a meta-character, we need to escape it for literal search using “\\” as given below.

Pattern pattern = Pattern.compile("Java \\(");

Matcher matcher = pattern.matcher("C++ (100), Java (99), C (89)");

while(matcher.find()) {

System.out.println("Matched \"" + matcher.group() + "\"");

}

Output

1	Matched "Java ("

Another way to fix this exception is to using Pattern.Literal constant while creating the pattern object. In this way, we do not need to escape any meta-characters in our pattern.