PatternSyntaxException: Dangling meta character even though no metacharacters are dangling

Question

I am trying to check if a string matches the pattern of a complex number with the format x+yi or x-yi. My regex for this purpose is ^\d+(\.\d+)?[-\+]\d+(\.\d+)?i and it works just fine on regexr (regexr.com/572bb).

Here is my minimal example in java:

public class ExampleClass {
    public static void main(){
        String comp = "3+5i";
        System.out.println(comp.matches("^\\d+(\\.\\d+)?[-\\+]\\d+(\\.\\d+)?i"));
    }

It throws the above exception. Here it is in full:

Exception in thread "main" java.util.regex.PatternSyntaxException: Dangling meta character '+' near index 0

The "+" it's complaining about is this one: [-\\+] but it doesn't seem like it's dangling since I escaped it. I am a beginner, so I would appreciate any help I can get. Thank you!

It's working for me. Please see code [**here**](https://onlinegdb.com/Bk_Qt0qp8). — , Jun 20 '20 at 00:33
@Mandy8055 I'm trying to understand this part: `[-+\\\\]` Doesn't this just say "allow as many '-' as you want" and then it escapes two semicolons? I'm very confused. — gowner, Jun 20 '20 at 00:33
@Mandy8055 I'm sorry for probably being annoying but the way I learned about regex, I thought square brackets means "match any character in the set" and in order to allow the "+" character, I have to escape it because it's a metacharacter. Does regex work differently in Java or am I just being kind of dumb? EDIT: didn't see your edit :D — gowner, Jun 20 '20 at 00:38
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/216310/discussion-between-flo-klar-and-mandy8055). — gowner, Jun 20 '20 at 00:42
You don't need to escape. `"^\\d+(\\.\\d+)?[-+]\\d+(\\.\\d+)?i"` will work. — , Jun 20 '20 at 00:57
Other than missing an end semi-colon, it worked fine for me. — WJS, Jun 20 '20 at 02:28
You don't want an optional minus sign before the real part of the number? — Cary Swoveland, Jun 20 '20 at 03:11

xxce10xx · Answer 1 · 2020-06-20T01:22:31.710

-1

To represent complexes x + yi, x-yi, you could do it like this

String comp = "3+5i";
System.out.println(comp.matches("^[0-9]+([.][0-9]+)?[+,-][0-9]+([.][0-9]+)?[i]$"));

edited Jun 20 '20 at 01:22

answered Jun 20 '20 at 00:24

xxce10xx

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Thank you for your help! Unfortunately, your solution doesn't allow me to input doubles or multi-digit numbers like "283.482+28.382i". – gowner Jun 20 '20 at 00:30
It also matches `"3,5i"`. – Jun 20 '20 at 00:54
@Flo Klar, You are right, I did not think about the floating numbers, the expression would be `^[0-9]+([.][0-9]+)?[+,-][0-9]+([.][0-9]+)?[i]$` – xxce10xx Jun 20 '20 at 01:20

score -1 · Answer 2 · edited Jun 20 '20 at 09:10

-1

The issue was nothing to do with that expression at all. I used String.split("+"); elsewhere in my code and neglected to escape that "+".

For posterity: Escape metacharacters when using String.split();.

edited Jun 20 '20 at 09:10

Mark Rotteveel

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answered Jun 20 '20 at 01:09

gowner

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PatternSyntaxException: Dangling meta character even though no metacharacters are dangling

2 Answers2