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POHow to iterate over regexp compliant strings
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  1. POHow to iterate over regexp compliant strings
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    copied!<p>What is the easiest way to implement a class (in Java) that would serve as an iterator over the set of all values which conform to a given regexp?</p> <p>Let's say I have a class like this:</p> <pre><code>public class RegexpIterator { private String regexp; public RegexpIterator(String regexp) { this.regexp = regexp; } public abstract boolean hasNext() { ... } public abstract String next() { ... } } </code></pre> <p>How do I implement it? The class assumes some linear ordering on the set of all conforming values and the next() method should return the i-th value when called for the i-th time.</p> <p>Ideally the solution should support full regexp syntax (as supported by the Java SDK).</p> <hr> <p>To avoid confusion, please note that the class is not supposed to iterate over matches of the given regexp over a given string. Rather it should (eventually) enumerate all string values that conform to the regexp (i.e. would be accepted by the matches() method of a matcher), without any other input string given as argument.</p> <hr> <p>To further clarify the question, let's show a simple example.</p> <pre><code>RegexpIterator it = new RegexpIterator("ab?cd?e"); while (it.hasNext()) { System.out.println(it.next()); } </code></pre> <p>This code snippet should have the following output (the order of lines is not relevant, even though a solution which would list shorter strings first would be preferred).</p> <pre><code>ace abce ecde abcde </code></pre> <hr> <p>Note that with some regexps, such as <code>ab[A-Z]*cd</code>, the set of values over which the class is to iterate is ininite. The preceeding code snippet would run forever in these cases.</p>
 

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