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Solution – java.util.regex.PatternSyntaxException: Dangling meta character near index 0

The java.util.regex.PatternSyntaxException: Dangling meta character near index 0 example shows the reasons for this exception. It also shows how to fix it using various solutions.

What is java.util.regex.PatternSyntaxException: Dangling meta character near index 0 and what causes it?

As the exception message suggests, if we try to match a meta-characters in a literal way in a regex pattern without escaping it, Java throws this exception. What is a meta-character? Well, in a regular expression, certain characters have a special meaning, for example, “+” (plus sign) matches the previous pattern one or more times. We have to escape these characters before we can match these in a literal way.

Here are some of the reasons for this exception to occur and their fixes.

Reason 1: Trying to split a string using one of the meta-characters

You are trying to split a string using the Java String split method using one of the regular expression meta-characters.

Consider below given example where we are trying to split the string content using the “+” sign.

Output

Solution:

Since the split method accepts a regex pattern, we need to escape the meta-characters using “\\” before matching them literally. These characters include “+”, “*”, “^”, “|”, and others.

Output

Reason 2: Trying to replace substring having a meta-character using the replaceAll method

Just like the split method, the Java String replaceAll method also accepts the regex pattern. Trying to match specific meta-character in a literal way without escaping it causes the same exception as given below.

Output

Solution:

Escape all meta-characters using “\\” in the replaceAll method as below.

Output

Reason 3: Problem with the regular expression pattern

The regex pattern we have mentioned has a problem. Consider below given example where I am not trying to match any meta-character literally, but still, the code will throw the exception due to incorrect usage of the “*” meta-character.

Output

Solution:

The solution is to fix the regular pattern. In the above example, the usage of the “*” is incorrect. The “*” matches the previous pattern zero or more times. Since the “*” is mentioned at the start of the pattern, there is no previous pattern or group to match zero or more times. Removing the * will fix the exception.

Important Note:

Not all unescaped meta-characters cause this exception. But when the intention is to match them literally, unescaped meta-characters will not produce the desired result. See the below example where we want to split the string by pipe (“|”), but the code does not produce the desired result.

Output

Well, that was not the desired output. The reason is, the pipe character is a meta-character in regex and we tried to match it literally without escaping it.

If you want to get the complete list of the java regex meta-characters, please visit Java Regular Expression Tutorial with Examples.

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

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