PatternSyntaxException: Illegal Repetition when using regex in Java

Question

I don't know much regex, but I need to match a simple pattern. The following should return true,

Pattern.matches("{\"user_id\" : [0-9]*}", inputLine)

when inputLine is

{"user_id" : 34}

However, I'm getting this exception:

java.util.regex.PatternSyntaxException: Illegal repetition
{"user_id" : 24}
    at java.util.regex.Pattern.error(Unknown Source)
    at java.util.regex.Pattern.closure(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.util.regex.Pattern.matches(Unknown Source)
    at org.whispercomm.manes.server.http.IntegrationTest.createUser(IntegrationTest.java:173)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
    at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:30)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

What am I doing wrong?

Answer 1

@IntelliJ IDE for example detects the inconsistency. Depending on the input, What you could do is to escape the characters or handle the problem using the data provided by the exception.

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

public class StackOverflow {
    private static String REGEX = "{\"user_id\" : [0-9]*}";

    public static void main(String[] args) {
        try {
            Pattern pattern = Pattern.compile(REGEX);

            // get a matcher object
            String input = "Hello stackoverflow";
            Matcher matcher = pattern.matcher(input);
            input = matcher.replaceAll("something");
        } catch (PatternSyntaxException e){
            System.out.println("PatternSyntaxException: ");
            System.out.println("Description: "+ e.getDescription());
            System.out.println("Index: "+ e.getIndex());
            System.out.println("Message: "+ e.getMessage());
            System.out.println("Pattern: "+ e.getPattern());
        }
    }
}

Output

PatternSyntaxException: 
Description: Illegal repetition
Index: 1
Message: Illegal repetition near index 1
{"user_id" : [0-9]*}
 ^
Pattern: {"user_id" : [0-9]*}

Possible solution

catch... ex
    StringBuilder sb = new StringBuilder(text);
    sb.insert(ex.getIndex() + 1, "\\");
    matcher.replaceAll(sb.toString())

Answer 2

The { and } are special in Java's regex dialect (and most other dialects for that matter): they are the opening and closing tokens for the repetition quantifier {n,m} where n and m are integers. Hence the error message: "Illegal repetition".

You should escape them: "\\{\"user_id\" : [0-9]*\\}".

And since you seem to be trying to parse JSON, I suggest you have a look at Jackson.

Answer 3

There should be plus operator:

user_id : [0-9]+

Double apostrophes only when the string has to contain it.

When the string including curly brackets use:

\{user_id : [0-9]+\}