Make checkstyle require variable prefix - regex

Question

I would like all checkbox variables to begin with "ckx".
^.*JCheckBox((?! ckx).)*$
This works for variable declarations but matches definitions too.
E.g.
ckxSomeCheckbox = new JCheckBox();
Any idea how to prevent this code to be displayed as an error?
Valid examples:
private JCheckBox ckxTest = new JCheckBox();
public javax.swing.JCheckBox ckxTest;
Invalid examples:
private JCheckBox abcTest;
private JCheckBox abcckxTest;
ckxTest = new JCheckBox();
ckxTest = new JCheckBox("");

Answer 1

I would recommend using the RegexpMultiline check so that you catch cases where people insert line breaks into the code in inconvenient places.
The regex would be \bJCheckBox\b\s*(?!ckx[A-Z])\w+ (explanation / examples).
<module name="RegexpMultiline">
<property name="format" value="\bJCheckBox\b\s*(?!ckx[A-Z])\w+"/>
<property name="message" value="Missing prefix 'ckx' on checkbox variable"/>
<property name="fileExtensions" value="java"/>
</module>
Note that I took out the lines where the faulty identifier is at the start of the line, because those lines would not compile. Unless you have the variable declared earlier, in which case that would be where the issue was flagged.
ckxTest = new JCheckBox();
ckxTest = new JCheckBox("");
Checking this with a regex isn't perfect, because people can use comments or subclasses to confuse the detector. So it would be a more powerful approach to use a PMD XPath rule or something, but that's for another question. :-)
The same could be achieved using RegexpSinglelineJava, which adds the ability to ignore comments, but would be derailed by unexpected line breaks:
<module name="RegexpSinglelineJava">
<property name="format" value="\bJCheckBox\b\s*(?!ckx[A-Z])\w+"/>
<property name="message" value="Missing prefix 'ckx' on checkbox variable"/>
<property name="ignoreComments" value="true"/>
</module>