Problems with Regex in Java

Question

In my current project I am confronted with the problem of having a string that can be constructed in 2 patterns. It consists of only one natural number, or 2 natural numbers with a '-' in between. I need to be able to distinguish between them. I would like to use the following code for this:

if (STRING.matches("*-*"))
{
    //Do something
} else {
    //Do something else
}

However, it gives me the following error:

Exception in thread "Thread-2" java.util.regex.PatternSyntaxException: Dangling meta character '*' near index 0

I also tried to put '#' at the beginning of the string ( of course I added it to the pattern), but this only caused the else part of the if-query to be executed.

Hope you can help me

isi_ko

Have you read the documetation for class java.util.regex.Pattern ? It explains java's regex syntax. — Abra
– Abra, Commented Apr 19, 2020 at 20:05

user · Accepted Answer · 2020-04-20 13:10:05Z

3

Your regex is incorrect because the asterisk is a metacharacter that needs a pattern before it. You can try something like this:

if (STRING.matches("[^-]*-[^-]*"))
{
    //Do something
} else {
    //Do something else
}

A better way would probably to use String#split("-") and put that into an array. Then you could check the length of that array to see how many numbers there and then use Integer.parseInt on each substring to obtain a natural number.

Another way would be to use a Matcher to find multiple groups. If (\d)* is the regex that matches a natural number, you can do this:

Pattern pattern = Pattern.compile("(\\d)*");
Matcher matcher = pattern.matcher(input);
matcher.find();
String first = matcher.group();
if (matcher.find()) {
  String second = matcher.group(); //this means there are two numbers
}

edited Apr 20, 2020 at 13:10

answered Apr 19, 2020 at 20:02

user

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1 Comment

isi_ko Over a year ago

Thanks that worked :D (Edit: I used the version with String.split("-");)

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