Java Regex not matching properly

Question

I need to check if a string has any equals sign on its own. My current regex does not seem to work within Java, even though RegexPal matches it.

My current code is:

String str = "test=tests";
System.out.println(str + " - " + str.matches("[^=]=[^=]"));

In the following test cases the first should be matched, the second shouldn't:

test=tests // matches t=t
test==tests // doesn't match

Regex Pal does it right, however, Java for some reason returns false for both test cases. Am I going wrong somewhere?

Thanks!

kennytm · Accepted Answer · 2012-11-25 19:16:18Z

6

Java's String.matches function matches the entire string, instead of just one part. That means, it is roughly equivalent to the regex ^[^=]=[^=]$, so both returns false. To build a regex working equivalent to yours, you should use:

str.matches("(?s).*[^=]=[^=].*")

(The (?s) ensures the . matches everything.)

Alternatively, you could build a Pattern and use Matcher for greater flexibility. This is what String.matches uses.

final Pattern p = Pattern.compile("[^=]=[^=]");
final Matcher m = p.matcher(str);
return m.find();

edited Nov 25, 2012 at 19:16

answered Nov 25, 2012 at 19:11

kennytm

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2 Comments

GManz Over a year ago

Aah I see! This makes sense as earlier it matches begin and end of a string even though I hadn't added ^ and $! Thanks!

J-Cake Feb 26 at 17:28

Why is this the behaviour? If I wanted it to match the whole string I would put start and end markers on it myself. Is there a reason I'm not seeing here?

Olaf Dietsche · Accepted Answer · 2012-11-25 19:07:58Z

1

You must add either * or + to your regexp:

str.matches("[^=]+=[^=]+")

This is needed, because string.matches() is anchored by default. This is equivalent to ^[^=]+=[^=]+$ and means that it must match the whole string and not only a part of it.

answered Nov 25, 2012 at 19:07

Olaf Dietsche

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Comments

Rohit Jain · Accepted Answer · 2012-11-25 19:14:18Z

1

You are just matching single character before and after =. Now, since String#matches tries to match the complete string with the given Pattern, so you need to ensure that you cover complete string in it. Obviously, your string does contain many characters before and after =.

So, to specify this, you need to use a quantifier, here: -

+ to match one or more characters
* to match zero or more characters

So, you would need to change your regex to: -

System.out.println(str + " - " + str.matches("[^=]+=[^=]+"))

edited Nov 25, 2012 at 19:14

answered Nov 25, 2012 at 19:08

Rohit Jain

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Daniel S. · Accepted Answer · 2012-11-25 19:15:07Z

1

The problem is that String.matches() does not search for a matching subsequence, but instead tries to match the complete string against your pattern.

For your purpose, you need Matcher.find(): http://docs.oracle.com/javase/6/docs/api/java/util/regex/Matcher.html#find()

answered Nov 25, 2012 at 19:15

Daniel S.

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