Conditional Regular Expression in Java?

Question

I have a conditional regular expression that works on regex test websites, such as regexlib.com, but cannot get it to work in my Java application.

But, http://www.regular-expressions.info/conditional.html indicates that Java doesn't support conditionals, but I've seen other posts on SO imply that it does.

An example of my RegEx is: (?(?=^[0-9])(317866?)|[a-zA-Z0-9]{6}(317866?))

It should match either of these inputs: 317866 or 317866A12 or FCF1CS317866

How do I work around this Java limitation?

TIA

Should it match 31786? If not, what is that ? doing after the last 6? — Mark Byers
– Mark Byers, Commented Sep 10, 2010 at 20:42

Eugene Kuleshov · Accepted Answer · 2010-09-10 20:07:17Z

11

Conditional expressions are not supported by java.util.regex.Pattern class. To get around that you could use a 3rd party regexp library such as JRegex

answered Sep 10, 2010 at 20:07

Eugene Kuleshov

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Mark Byers · Accepted Answer · 2010-09-10 20:07:29Z

1

How about just doing this instead?

(?:[a-zA-Z0-9]{6})?(317866?)

Or if you know that the longer version always start with a letter then you can use this:

(?:[a-zA-Z][a-zA-Z0-9]{5})?(317866?)

It will first try to match 6 alphanumerics followed by 31786 or 317866, and if that fails it will then backtrack and try matching 31786 or 317866.

answered Sep 10, 2010 at 20:07

Mark Byers

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6 Comments

RNeuendorff Over a year ago

Thanks for the quick replies... I'm not real familiar with RegEx and I didn't realize it could be that simple. My business rules are: if it starts with an alpha, ignore the first 6, if it starts with a numeric, start the match from the beginning. So, the second example seems to be more complete.

Mark Byers Over a year ago

@RNeuendorff: The second example should work and it will not match 123456316866, which the first expression will match. I think you want the second example.

RNeuendorff Over a year ago

@Mark Byers: That regex is not completely accurate. I do not want to match something like "AB1363183A23", but that regex does.

Mark Byers Over a year ago

@RNeuendorff: Are you sure about that? The string you gave as an example doesn't even contain 31786 so how could it match?

Mark Byers Over a year ago

@RNeuendorff: The best test is to write a short Java program to test them. It takes only about 3 or 4 lines of code.

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