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POCapturing <thisPartOnly> and (thisPartOnly) with the same group
 

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    <p>Let's say we have the following input:</p> <pre><code>&lt;amy&gt; (bob) &lt;carol) (dean&gt; </code></pre> <p>We also have the following regex:</p> <pre><code>&lt;(\w+)&gt;|\((\w+)\) </code></pre> <p>Now we get two matches (<a href="http://www.rubular.com/r/nfwk7d5YRG" rel="nofollow noreferrer">as seen on rubular.com</a>):</p> <ul> <li><code>&lt;amy&gt;</code> is a match, <code>\1</code> captures <code>amy</code>, <code>\2</code> fails</li> <li><code>(bob)</code> is a match, <code>\2</code> captures <code>bob</code>, <code>\1</code> fails</li> </ul> <p>This regex does most of what we want, which are:</p> <ul> <li>It matches the open and close brackets properly (i.e. no mixing)</li> <li>It captures the part we're interested in</li> </ul> <p>However, it does have a few drawbacks:</p> <ul> <li>The capturing pattern (i.e. the "main" part) is repeated <ul> <li>It's only <code>\w+</code> in this case, but generally speaking this can be quite complex, <ul> <li>If it involves backreferences, then they must be renumbered for each alternate!</li> <li>Repetition makes maintenance a nightmare! (what if it changes?)</li> </ul></li> </ul></li> <li>The groups are essentially duplicated <ul> <li>Depending on which alternate matches, we must query different groups <ul> <li>It's only <code>\1</code> or <code>\2</code> in this case, but generally the "main" part can have capturing groups of their own!</li> </ul></li> <li>Not only is this inconvenient, but there may be situations where this is not feasible (e.g. when we're using a custom regex framework that is limited to querying only one group)</li> </ul></li> <li>The situation quickly worsens if we also want to match <code>{...}</code>, <code>[...]</code>, etc.</li> </ul> <p>So the question is obvious: <strong>how can we do this without repeating the "main" pattern?</strong></p> <blockquote> <p>Note: for the most part I'm interested in <code>java.util.regex</code> flavor, but other flavors are welcomed.</p> </blockquote> <hr> <h3>Appendix</h3> <p>There's nothing new in this section; it only illustrates the problem mentioned above with an example.</p> <p>Let's take the above example to the next step: we now want to match these:</p> <pre><code>&lt;amy=amy&gt; (bob=bob) [carol=carol] </code></pre> <p>But not these:</p> <pre><code>&lt;amy=amy) # non-matching bracket &lt;amy=bob&gt; # left hand side not equal to right hand side </code></pre> <p>Using the alternate technique, we have the following that works (<a href="http://www.rubular.com/r/ojoknCda2A" rel="nofollow noreferrer">as seen on rubular.com</a>):</p> <pre><code>&lt;((\w+)=\2)&gt;|\(((\w+)=\4)\)|\[((\w+)=\6)\] </code></pre> <p>As explained above:</p> <ul> <li>The main pattern can't simply be repeated; backreferences must be renumbered</li> <li>Repetition also means maintenance nightmare if it ever changes</li> <li>Depending on which alternate matches, we must query either <code>\1 \2</code>, <code>\3 \4</code>, or <code>\5 \6</code></li> </ul>
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    1. USPeter Boughton
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    1. COMy thought has always been that if the problem requires an appendix, maybe it's a problem best not solved through regex.
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      1. USMark Peters
    2. CO@Mark: for the most part this is a self-study, not an on-the-job scenario. For the main part of the question I used the simplest example; the appendix is more complex, but emphasizes the points stronger, which is why I thought it's worth including.
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